Transient heat transfer in vertical ground heat exchangers

Transient heat transfer inside and in the vicinity of vertical ground heat exchangers is the main focus of the present thesis.;A hybrid analytical-numerical one-dimensional model is presented where heat transfer in the borehole is treated numerically and ground heat transfer is handled with the classic cylindrical heat source analytical solution. The one-dimensional approach imposes several assumptions which are rigorously presented. As well, the model requires careful treatment of the various time periods from the residence time of the fluid in the borehole to the energy simulation time and including the time steps of the numerical simulation. The hybrid model is successfully validated against analytical solutions and field data. It is used in simulations over an entire heating season with a single-stage geothermal heat pump linked to a borehole. Two sets of simulations are performed: with and without borehole thermal capacity. Results show that for a typical borehole, the annual heat pump coefficient of performance (COP) can be underestimated by 4 to 4.6% when the borehole simulations do not account for the grout and fluid thermal capacities.;A significant level of effort went into the design, construction, and commissioning of a small-scale (1/100) experimental sand tank to study transient heat transfer in the vicinity of boreholes. The main features of the facility include: i) an instrumented 1.23 m long borehole with a carefully positioned U-tube and filled with well-characterized small glass beads which act as the grout; ii) a string rack instrumented with some 60 calibrated thermocouples precisely located for sand temperature measurement; iii) laboratory-grade sand with known thermal properties; iv) fluid conditioning equipment that allow to feed the facility with user-specified inlet temperature and flow rate.;The experimental facility proved to be invaluable for validating a two-dimensional axi-symmetric numerical model developed for this study. Comparison results show that the numerical results are in very good agreement with the experimental data with most of the results lying within the experimental uncertainty. The measured azimuthal temperature variation at the borehole wall seems to corroborate recent findings obtained using the analytical multipole method.