Georgia Radioti

Thermal Response Test in Borehole Heat Exchangers Equipped with Fiber Optics

Four double-U borehole heat exchangers (BHEs) of 100m long were installed on the campus of the University of Liege (Liege, Belgium). The installation procedure and technical difficulties are presented. Fiber optic cables are attached along the length of one pipe loop in each BHE. Temperature is measured along the fibers based on the fiber optic distributed temperature sensing (DTS) technique. Thermal response test (TRT) is conducted in order to determine the rock thermal properties. The DTS instrument records the temperature evolution along the pipe loop during the TRT. Rock thermal conductivity through depth can be estimated based on the recorded data. A 3D model is developed using the finite element code LAGAMINE in order to simulate the TRT. The accuracy of the numerical model is improved by simulating the potential variation of the rock thermal conductivity.

Fiber-optic Temperature Profiles Analysis for Closed-loop Geothermal Systems - A Case Study

SUMMARY In order to study the behaviour of shallow closed-loop geothermal systems four borehole heat exchangers equipped with fiber optics were installed on the campus of the University of Liege (Liege, Belgium) over a surface area of 32m². This paper presents the analysis of continuous, high-resolution temperature profiles measured along the boreholes length. The undisturbed ground temperature measurements indicate heat loss from ground structures located close to the boreholes. A 3D numerical model is presented to reproduce the measured temperature profiles. Temperature profiles during hardening of the grouting material indicate extended fractured zones in the rock mass. Temperature measurements during the recovery phase of a Distributed Thermal Response Test indicate the succession of rock layers with different mineral content. The results are in good agreement with those of the borehole televiewer logging method. The presented analysis could provide information on bedrock heterogeneity, on the anisotropic thermal behaviour of the rock mass and on the ground temperature variations due to heat loss from ground structures. These information could significantly contribute to the long-term behaviour prediction of the geothermal system and the geothermal reservoir potential.

A Case Study of a Long-Duration Thermal Response Test in Borehole Heat Exchangers

Shallow closed-loop geothermal systems are worldwide applied providing economical and environmental benefits. This paper presents an in-situ study of four Borehole Heat Exchangers of 100 m long, installed in an heterogeneous bedrock in the campus of the University of Liege (Liege, Belgium). A Thermal Response Test (TRT) of a heating phase of 7 months was conducted in one of the boreholes. During this test, temperature was measured at the pipe inlet and outlet, as well as along the four boreholes by the fiber optics. To further investigate the measured data, the test was simulated by 3D numerical modeling. The comparison of the measured data with the numerical results allowed to detect the critical parameters for the behavior of the BHE and for the temperature evolution in the surrounding rock mass. In this case study, the behavior of the BHE could be predicted based on the results of a typical-duration TRT (of a few days), considering the ground an homogenous and isotropic material. However, the thermal plume in the surrounding ground seems to be influenced by several factors, such as the bedrock heterogeneity, the distance to the heating source, air temperature variations and thermal effects at the borehole bottom end.