Distributed Thermal Response Tests Using a Heating Cable and Fiber Optic Temperature Sensing

Abstract

Thermal response tests are used to assess the subsurface thermal conductivity to design ground-coupled heat pump systems. Conventional tests are cumbersome and require a source of high power to heat water circulating in a pilot ground heat exchanger. An alternative test method using heating cable was verified in the field as an option to conduct this heat injection experiment with a low power source and a compact equipment. Two thermal response tests using heating cable sections and a continuous heating cable were performed in two experimental heat exchangers on different sites in Canada and France. The temperature evolution during the tests was monitored using submersible sensors and fiber optic distributed temperature sensing. Free convection that can occur in the pipe of the heat exchanger was evaluated using the Rayleigh number stability criterion. The finite and infinite line source equations were used to reproduce temperature variations along the heating cable sections and continuous heating cable, respectively. The thermal conductivity profile of each site was inferred and the uncertainly of the test was evaluated. A mean thermal conductivity 15% higher than that revealed with the conventional test was estimated with heating cable sections. The thermal conductivity evaluated using the continuous heating cable corresponds to the value estimated during the conventional test. The average uncertainly associated with the heating cable section test was 15.18%, while an uncertainty of 2.14% was estimated for the test with the continuous heating cable. According to the Rayleigh number stability criterion, significant free convection can occur during the heat injection period when heating cable sections are used. The continuous heating cable with a low power source is a promising method to perform thermal response tests and further tests could be carried out in deep boreholes to verify its applicability.

Evaluation of Subsurface Heat Capacity through Oscillatory Thermal Response Tests

“…Energies 2021, 14, 5791 6 of 26 4. Analysis of the recovery period to estimate TCcooling, which we assume as the in-situ subsurface TC because it is not affected by the borehole thermal resistance ( [22], Figure 2D); 5.…”

“…The test was performed with a heating cable unit and a cable length of 22.5 m [3]. The heating cable technique was used because it is an affordable, easy and sound technique to infer the underground thermal conductivity as already demonstrated in the literature [3][4][5]. Submersible temperature sensors (accuracy 0.1 • C, resolution 0.032 • C) were placed along the heating cable at depths of 2.5, 5, 10, 12.5, 15, 17.5, 20 and 22.5 m from the ground level.…”

“…On the other hand, borehole thermal resistance cannot be properly assessed with a heating cable test. Other than being very compact and needing only 120 V power, the heating cable unit can provide a TC profile of the ground with several temperature sensors at depth, or with a fiber-optic cable [4,5]. Moreover, it does not require a BHE since it can be performed in open wells, provided that water is present to ensure thermal contact with the subsurface.…”

“…Moreover, it does not require a BHE since it can be performed in open wells, provided that water is present to ensure thermal contact with the subsurface. Electric cable with heating and non-heating sections were also tested, but significant free convection occurs in the pipe (or well) according to the Rayleigh number stability criterion, thus allowing only 15% accuracy in TC estimation [4]. TRTs are commonly carried out for 48 to 72 h with a constant power ( [2] and references therein).…”

Numerical and Experimental Validation of the Applicability of Active‐DTS Experiments to Estimate Thermal Conductivity and Groundwater Flux in Porous Media

“…Active‐DTS experiments have thus been conducted in sealed boreholes (Coleman et al., 2015; Maldaner et al., 2019; Munn et al., 2020; F. Selker & J. S. Selker, 2018) and other promising studies proposed the direct deployment of FO cables vertically into unconsolidated sedimentary aquifers using direct‐push equipment (Bakker et al., 2015; des Tombe et al., 2019). Concurrently, active‐DTS methods were largely developed and applied in unsaturated soils, offering the possibility of estimating the soil water content and thermal properties (Benitez‐Buelga et al., 2014; He, Dyck, Horton, Li, et al., 2018; He, Dyck, Horton, Ren, et al., 2018; Sayde et al, 2010, 2014; Weiss, 2003; Wu et al., 2019) or conducting distributed thermal response test for geothermal energy applications (Vélez Márquez et al., 2018; Zhang et al., 2020).…”