Transient Heat Transfer in Boreholes with Application to Non-Grouted Borehole Heat Exchangers and Closed Loop Engineered Geothermal Systems

This thesis concerns heat transfer processes in single boreholes and systems of boreholes. The thesis is divided in two parts of which the first considers borehole heat exchangers (shallow geothermal energy), and the second part deals with heat transfer in an engineered geothermal system, made up of boreholes forming a closed loop system (deep geothermal energy). The essence of the thesis is described hereunder, starting with the borehole heat exchangers. The borehole heat exchangers (BHEs) are used in ground source heat pump (GSHP) systems as a source and sink for thermal energy, the U- tube configuration of BHEs is the most common. In this thesis the heat transfer processes in U-tube BHEs are studied with the use of both numerical and analytical models. A novel numerical model for the heat transfer in non-grouted BHEs (which is common in Norway and Sweden) has been developed. The model includes a correlation that accounts for the occurrence of natural convection in the water surrounding the collector. The model is compared with experimental data from distributed temperature measurements obtained during both a distributed thermal response test (heat injection) and during heat pump operation (heat extraction). The model is found to accurately replicate the experimental data. The model is used to analyze the experimental data and to gain further understanding for the heat transfer processes in non-grouted BHEs. A borehole thermal energy storage (BTES) using a given solar collector as the means of thermal recharge is studied. The BTES is operated with two individually, but thermally interacting circuits with different thermal loads, were only one of the circuits is thermally recharged during the warm season. The system is studied using an analytical model which allows for individual thermal loads in the different boreholes of the BTES. The system is found to have a marginally better performance as compared to the alternative of applying thermal recharge to all BHEs. With access to more energy for thermal recharge it would be better to recharge both circuits. The coaxial BHE is an alternative to the conventional U-tube BHE, and it has in general a lower internal thermal resistance, which improves the performance of the GSHP system. In addition, it is more suitable for deep boreholes since for a given borehole dimension it can allow for a larger flow area, and thus for a larger mass flow and / or a lower pressure drop. A numerical model is developed for the co