Shallow geothermal energy: effect of in-situ conditions on borehole heat exchanger design and performance

The in-situ conditions are critical for the performance of Borehole Heat Exchangers (BHEs). However, in practice they are often not adequately considered, overwhelming the potential of these systems. This thesis focuses on the accurate estimation of the in-situ characteristics and on their influence on the design and the behaviour of BHEs based on an in-situ study of an heterogeneous bedrock in a semi-urban environment (campus of the University of Liege, Liege, Belgium). The experimental site consists of four double-U BHEs, of about 100 m long, installed over a surface area of 32 m² and equipped with fiber optic cables. Several temperature measurements and Distributed Thermal Response Tests (DTRTs) were conducted in situ in a period of four years, including a long-duration DTRT (heating phase of 7 months), during which temperature was measured by the fiber optics in all the four boreholes. These measurements create a unique data set, that allows to investigate the BHE behaviour for longer heating periods, to study the effect of various factors on the temperature field evolution at the heterogeneous bedrock at the in-situ scale and to evaluate the contribution of temperature borehole logging to the optimisation of BHEs.
The effect of urbanisation is studied based on the in-situ measurements and on 3D numerical modelling and its influence on the design is expressed in terms of the maximum extracted power. The subsurface characteristics are correlated with the measured fiber optic profiles and the potential of temperature borehole logging for optimising the design of BHEs in practise is presented. The accuracy of the thermal response test results in the case of insufficient test rig insulation is investigated and recommendations are provided regarding the interpretation of the data by the widely applied Infinite Line Source model. The in-situ measurements during the long-duration DTRT are presented and analysed, together with a 3D numerical model of the test. In this case-study, the possible variation of the effective thermal conductivity along the layers and the air temperature variations during the test do not seem to have a dominant effect on the BHE behaviour during the whole heating phase. The controlling factors for the temperature field evolution in the surrounding rock mass (bedrock heterogeneity, the air temperature variations, the distance to the heating source and the thermal effects at the borehole bottom end) are detected in the measured profiles and their influence is discussed.