Co-Axial, Closed Loop Geothermal Systems Modelling

Abstract

The Earth’s interior contains considerable unused thermal energy, far exceeding the current global energy demand. Using shallow geothermal sources for direct heating is a historically viable option. This thesis explored the exploitation of deeper sources with the use of a system of Coaxial Borehole Heat Exchangers (CBHEs). The CBHE system would be used to heat the northeast corner of the University of Calgary's main campus using horizontal wells. A computer model was developed to simulate the subsurface operation of a single CBHE. It utilized COMSOL Multiphysics 6.1, which solved the fluid flow and heat transfer physics equations using the finite element method. The CBHE’s geometry consists of a 2000 m deep well with a 2000 m lateral. The inner tubing of the well was seated concentrically within the outer casing. Various assumptions were made to simplify the model to decrease computation time and ease the convergence of a solution. These include modelling the entire geometry as one long axisymmetric system in two dimensions, neglecting the effects of the area between the bottom of the tubing and the bottom of the well, and neglecting the gravity effects. The model's accuracy and its assumptions were verified against experimental data and analytical derivations. The system's sensitivity to parameters such as the radii of the borehole and tubing, the fluid flow rate, tubing insulation properties, the number of wells, and the bottom hole temperature were investigated. Variations in these parameters directly influenced the injected and produced temperatures, impacting the system's thermal performance. One major finding was that when reinjecting unused heat, the CBHE performs better when the inner tubing is uninsulated. However, when keeping the injection temperature constant, the thermal performance was optimal with fully insulated tubing. Additionally, the time until thermal interaction between wells was explored by investigating their radial spacing; the time was found to be directly proportional to the square of the radial well spacing. Overall, a system of 44 CBHEs can provide the NE corner of the university campus with the necessary thermal power to match the 2019 usage.