Novel methodology for measuring groundwater flow across borehole heat exchangers

Michalski, Alexander Jan; Clauser, Christoph (Thesis advisor); Amann, Florian (Thesis advisor); Villinger, Heinrich (Thesis advisor)

Aachen : RWTH Aachen University (2021, 2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

Groundwater flow measurement can be a crucial parameter for the efficiency and operation of a geothermal heat exchanger array, because advective heat transport can influence the temperature distribution of neighboring borehole heat exchangers (BHE). Therefore, knowledge of the hydrological parameters of the subsoil is of great importance for its operation and is imperative for a meaningful simulation of the long-term behavior of its essential parameters. In the present work, a new measurement method for determining the groundwater flow directly on the outer wall of borehole heat exchangers is presented. As a prerequisite for the new method, it is necessary to develop a measuring tool with which the temperature field in the immediate vicinity of the borehole heat exchangers can be measured, because the temperature distribution is directly dependent on the direction of the groundwater flow and its strength. For this purpose, the temperature field is measured in horizontal planes around the borehole heat exchangers using digital temperature sensors. In particular, no additional marking material or temperature source should be used and the flow field at the borehole heat exchangers should not be influenced. The work builds on three publications, which are structured as follows. The first publication is a patent and summarizes the idea and its application. The technical details from data acquisition to evaluation are discussed. The individual components are listed and described here. The temperature sensor modules (TSM) are built in horizontal planes consisting of temperature sensor rings inside and outside on a borehole heat exchanger. This enables the temperature distribution to be measured on the fluid pipes in the filling area of the BHE and on its outer wall. Digital temperature sensors are used to enable a high degree of flexibility and cascading of such horizontal measurement levels. The electronic components are designed to be as simple as possible in order to establish an inexpensive measuring method. In the first scientific publication, a sensitivity study of the measurement method and its experimental verification is presented. For this purpose, a sandpit test rig was built in which a prototype of the temperature sensor module can be tested with a section of borehole heat exchangers under real conditions. Two water tanks on the sandpit are used for adjusting groundwater flow in various strengths using a hydraulic gradient. The experimental data are compared with numerical simulation results. A horizontal section of the test setup is numerically simulated in 2D using the finite difference method and the results are verified against the experiment. The numerical sensitivity study with regard to the direction of the groundwater flow and the groundwater flow rate determines the resolution of the new measurement method. The result is a parameter range in which the new measurement method is valid. In the second published scientific publication, the new measurement method is applied in an existing borehole heat exchanger field and the results are compared with the results of a commercial optical well measurement method. A testbed borehole heat exchanger with temperature sensor modules has been equipped and put into operation at the geothermal probe field of the E.ON Energy Research Center at RWTH Aachen University. The temperature sensor modules were installed according to a previously performed Enhanced Geothermal Response Test (EGRT) at the hydrologically relevant depths in which advective heat transfer is suspected. In a test run, the temperature distribution on and in the borehole heat exchangers is measured. The groundwater flow parameters are determined from numerical simulations using the methodology from the first publication. The numerical simulation results are in good agreement with the advective dominated areas of heat transport from the EGRT. In comparison to the optical method according to Schöttler (2000), there is a similar order of magnitude and the same direction for the groundwater flow. In conclusion, the new measuring method presented here is suitable for determining groundwater flow from the temperature distribution of the near field of a borehole heat exchanger. Groundwater flow can be measured with the method presented here up to a speed of a few meters per year and is therefore more accurate than most conventional measurement methods. The radially fine-resolution measurement of the temperature distribution inside and outside the BHEs also allows the thermal conditions inside a borehole heat exchanger to be studied.