Time-lapse electrical resistivity imaging of the thermally affected zone of a Borehole Thermal Energy Storage system near Torino (Northern Italy)

Abstract

A Borehole Thermal Energy Storage living lab was built up nearby Torino (Northern Italy). This living lab aims at testing the ability of the alluvial deposits of the northwestern Po Plain to store the thermal energy collected by solar thermal panels and the efficiency of energy storage systems in this climatic context. Different monitoring approaches have been tested and analyzed since the start of the thermal injection in April 2014. Underground temperature monitoring is constantly undertaken by means of several temperature sensors located along the borehole heat exchangers and within the hydraulic circuit. Nevertheless, this can provide only pointwise information about underground temperature distribution. For this reason, a geophysical approach is proposed in order to image the thermally affected zone (TAZ) caused by the heat injection: surface electrical resistivity measurements were carried out with this purpose. In the present paper, results of time-lapse daily acquisitions are reported with the aim of imaging the thermal plume evolution within the subsoil. Resistivity data, calibrated on local temperature measurements, have shown their potentiality in imaging the heated plume of the system and depicting its evolution within the day. Different types of data processing were adopted in order to face issues mainly related to a highly urbanized environment. The use of apparent resistivity proved to be in valid agreement with the results of different inversion approaches. The inversion processes did not significantly improve the qualitative and quantitative TAZ imaging in comparison to the pseudo-sections. This suggested the usefulness of apparent resistivity data alone for a rough monitoring of TAZ in this kind of applications.

Time-Lapse 3D Electric Tomography for Short-time Monitoring of an Experimental Heat Storage System

“…Particular focus is given to evaluate the reliability of ERT data with respect to the other two temperature evolution strategies independently. Due to the limited space available to perform ERT surveys and to the presence of anthropic disturbances (i.e., entry way), known to highly affect electrical signal at the site [32], the area and depth over which is possible to compare all of the measurements is reduced. Therefore, results are presented and discussed focusing on the zone surrounding the BHEs at the depth of 5 m·b.g.l.…”

“…For practical and construction reasons, the temperature sensors in the boreholes were attached to the plastic pipes of the heat exchangers. This can obviously affect the temperature recordings, that could show, during heat injection, temperatures higher than the real ground temperatures given that data are influenced by the HCF [32]. Data from MH are instead not disturbed by the presence of the HCF, but its distance from the boreholes makes it difficult to register a significant temperature change in a short-time window, such as the one considered in the present paper.…”

“…The results presented in this paper refers to a single injection day during the first working summer (31 July 2014). The wintertime circulation is not described here because considered not important for the purpose of the paper, further detailed information can be found in [31,32].…”

“…A range of 0.018 ÷ 0.025 • C −1 has been found by several authors for m [33,[44][45][46] and it varies according to the type of fluid and sediments. For the present case, an m value of 0.021 • C −1 was assumed after calibration tests at lab scale [27] and previous geophysical investigations at the field scale [32]. To obtain temperature differences images it is therefore possible to directly convert the resistivity difference (in percentage) of each voxel.…”

“…A 3D heat propagation numerical simulation, calibrated on the long-time plant behavior and on local temperature measurements of the plant, was used to validate the geophysical interpretation and to discuss strengths and limitations of the different approaches. The results obtained were finally compared with two recent literature works: Giordano et al [32], that evaluated the use of apparent and inverted resistivity data for imaging the TAZ of the same case study, but with a 2D acquisition and inversion approach; Lesparre et al [29], that carried out a methodology similar to the one presented here (including the use of numerical simulations), but on an open-loop system lying over a saturated heterogeneous alluvial aquifer. With respect to previous literature examples for which similar applications were presented (besides the two above mentioned see also [33] and following works) the present case study took advantage of a full 3D acquisition and processing of the resistivity data acquired with a surface electrodes disposition, which therefore limits the impact of the geophysical monitoring with respect to more invasive approaches (e.g., borehole ERT).…”