**4.2. Experimental results throughout a distributed thermal response test in a borehole heat exchanger**

In this section, an experimental test of the implemented system in one of the four BHE installed on the campus of the University of Liege (Liege, Belgium) is presented, with the aim of testing its performance and analyzing the obtained results.

The installation is equipped with double-U geothermal pipes of 100 m long, over a surface area of 32 m2 . Deposits of sand and gravel characterize the site geology until a depth of approximately 8 m. Then, the bedrock follows until the end of the borehole, which consists mainly of siltstone and shale inter-bedded with sandstone, while fractured zones are detected in the rock mass mostly until a depth of 35 m. In this installation, thermal behavior of fractured bedrock stratigraphy was being investigated throughout TRTs and distribution temperature sensing (DTS) technique. Hence, during the insertion of the geothermal pipes, fiber optics thermometers were tapped every 50 cm in direct contact with the outside part of the pipe wall. Given the relatively small borehole diameter (136 mm), spacers were not used during the installation and the distance between the U-legs is in the order of 3 cm. Among the four available boreholes, the test was conducted in B2, which was backfilled with a bentonitebased commercial material (Füllbinder, *λ* = 0 .95 W/mK).

Fiber optics makes possible to obtain continuous, high-resolution temperature profiles along the pipes length by applying the DTS technique [8]. The temperature resolution of the fiber optic measurements presented in this study (standard deviation) was in the order of 0.05°C. Temperature was recorded every 20 cm (sampling interval) with a spatial resolution of 2 m.

On 15 December 2015, an enhanced TRT of a heat injection of 2 kW and a duration of approximately 7 days was conducted in one of the single U-pipes of B2 which are disposed in a parallel configuration. The fiber optic cables were installed along the single U-pipe in which the heat was injected while the Geowire was inserted in the nonheated single U-pipe (observer pipe), which was filled with water. **Figure 29** presents a cross section of the borehole with the U-pipes, the location of the temperature measurement instruments where the heat was injected.

From the user interface, the Geowire temperature acquisition sequence was settled to lower the sensor until a depth of 40 m. Then, the sensor was established to stop every 0.5 m for a period of 5 s in order to achieve a thermal stabilization and avoid the possible convective effects, produced by the moving water when lowering the sensor. After 5 s, the device was programmed to record three samples of temperature, one every second and storage the average value in the specified database.

**Figure 29.** Cross-section of the BHE utilized in the experiment.

**4.2. Experimental results throughout a distributed thermal response test in a borehole** 

In this section, an experimental test of the implemented system in one of the four BHE installed on the campus of the University of Liege (Liege, Belgium) is presented, with the aim of testing

The installation is equipped with double-U geothermal pipes of 100 m long, over a sur-

of approximately 8 m. Then, the bedrock follows until the end of the borehole, which consists mainly of siltstone and shale inter-bedded with sandstone, while fractured zones are detected in the rock mass mostly until a depth of 35 m. In this installation, thermal behavior of fractured bedrock stratigraphy was being investigated throughout TRTs and distribution temperature sensing (DTS) technique. Hence, during the insertion of the geothermal pipes, fiber optics thermometers were tapped every 50 cm in direct contact with the outside part of the pipe wall. Given the relatively small borehole diameter (136 mm), spacers were not used during the installation and the distance between the U-legs is in the order of 3 cm. Among the four available boreholes, the test was conducted in B2, which was backfilled with a bentonite-

Fiber optics makes possible to obtain continuous, high-resolution temperature profiles along the pipes length by applying the DTS technique [8]. The temperature resolution of the fiber optic measurements presented in this study (standard deviation) was in the order of 0.05°C. Temperature was recorded every 20 cm (sampling interval) with a spatial resolu-

On 15 December 2015, an enhanced TRT of a heat injection of 2 kW and a duration of approximately 7 days was conducted in one of the single U-pipes of B2 which are disposed in a parallel configuration. The fiber optic cables were installed along the single U-pipe in which the heat was injected while the Geowire was inserted in the nonheated single U-pipe (observer pipe), which was filled with water. **Figure 29** presents a cross section of the borehole with the U-pipes, the location of the temperature measurement instruments where the

From the user interface, the Geowire temperature acquisition sequence was settled to lower the sensor until a depth of 40 m. Then, the sensor was established to stop every 0.5 m for a period of 5 s in order to achieve a thermal stabilization and avoid the possible convective

. Deposits of sand and gravel characterize the site geology until a depth

**heat exchanger**

152 Field - Programmable Gate Array

face area of 32 m2

tion of 2 m.

heat was injected.

its performance and analyzing the obtained results.

**Table 1.** Geowire distance interval measurement deviation.

**Distance interval Deviation** 0.5 m ±1 cm 50 m ±5 cm

based commercial material (Füllbinder, *λ* = 0 .95 W/mK).

**Figure 30** shows the Geowire adapted in a structure and the different components of the implemented system before the beginning of the test in the field.

The section of Graph & Charts of the web application allows the users to visualize the recorded data remotely during a real test. Once the device is running it is possible to observe real-time charts or load previously stored temperature profiles from the database. **Figure 31** shows the user interface for a temperature profile obtained during the announced test.

**Figure 32** presents the recorded temperature profile with the proposed system inside the observer pipe, and the registered fiber optic temperature measurements (along pipe inlet and pipe outlet outer surfaces) during the heating phase of the TRT. The oscillations observed in the recorded datasets along the observed pipe may be attributed to the varying distance through depth between the observer pipe and the U-heated pipe, as well as to the ground heterogeneity. Moreover, in the first ~18 m, temperature is also affected by the air temperature as previously studied by Radioti et al. [9].

The main advantage of the recorded temperature profiles is that, by applying the proposed procedure of Aranzabal et al. [10], it can contribute to calculate a detailed depth-dependent thermal conductivity profile of the BHE subsoil surrounding layers. Basically, it consists in an iterative simulation process of a numerical model in order to fit simulation results with experimental data.

**Figure 30.** The implemented system before the beginning of the acquisition process in a BHE.

**Figure 31.** Recorded temperature profile along the depth through the user interface.

**Figure 32.** Recorded vertical temperature profile by the Geowire and the fiber optics for 18 of December.
