## 2 Materials and Methods
### Materials
#### 2.2.2 Borehole Q-Thermie-Uniovi 1 and 2  
For this research, two geothermal boreholes were employed. The boreholes, referred to as Q-Thermie-Uniovi 1 and Q-thermie-Uniovi-2, are located within the Viesques' campus of the University of Oviedo (Gijon, Spain), 26 m above sea level and 3 km away from the coast. Their coordinates are longitude \(5^{\circ}37^{\prime}16^{\prime\prime}\) Wand latitude \(43^{\circ}31^{\prime}23^{\prime\prime}\) N.  
The selection of the experimental site and borehole characteristics was based on both technical and scientific criteria. The 7.3 m spacing between boreholes was chosen to replicate realistic configurations commonly found in small-to-medium-sized geothermal installations, where close borehole spacing can lead to significant thermal interference effects. The 48 m depth was selected to ensure that sensors were placed in a stable thermal region of the subsurface, reducing the influence of surface temperature variations. Additionally, the site was selected for its homogeneous geological composition, which facilitates the interpretation of the experimental results and the validation of the numerical model. This configuration allows for an accurate observation of heat propagation between the boreholes and supports the validation of the developed module under real and controlled conditions.  
The temperature measurements used for model validation were obtained using DS18B20 digital sensors, which have a manufacturer-specified accuracy of \(\pm\)0.5 \({}^{\circ}\)C and a resolution of 0.01 \({}^{\circ}\)C. Data were recorded every 10 min using an Arduino-based dualogger. Sensors were installed at multiple depths inside the borehole, and for this study, the sensor located at 22.62 m was selected. This depth lies below the convergence zone, where the temperature remains stable year-round and is not significantly affected by surface thermal fluctuations or groundwater level variations.  
Although potential sources of uncertainty such as sensor calibration drift, minor installation misalignments, or local thermal heterogeneities may be present, the impact of these factors was minimized through careful sensor placement and validation procedures. The recorded temperatures are therefore considered robust and suitable for validating the numerical model.  
Borehole Q-Thermie-Uniovi 1  
The drilled borehole has a diameter of \(\mathrm{D_{b}}=125\) mm and a depth of \(\mathrm{h}=48\) m. Two concentric pipes have been installed inside; the outer pipe has a diameter of \(\mathrm{D_{0}}=50\) mm, and the inner pipe has a diameter of \(\mathrm{D_{i}}=25\) mm. The length of the outer pipe is 48 m and it is closed at the bottom. The inner concentric pipe is open at the bottom, allowing the fluid descending through it to rise back to the surface through the annular section [20].  
Borehole Q-Thermie-Uniovi 2  
Unlike the previous borehole, this one is exposed to the elements and is located 7.3 m from the Q-Thermie-Uniovi 1 borehole. The borehole has a diameter of D\({}_{\text{b}}\) = 125 mm and a depth of h = 48 m. This borehole only has an exterior geothermal pipe with a diameter of D\({}_{0}\) = 50 mm, and its interior is filled with water [20].  
To monitor the temperature inside the borehole during the TRT, the following instrumentation was used:  
* Commercial temperature sensors (DS18B20) inside the pipe and connected by a 1-wire serial bus were placed. The sensors are positioned at different depths. The data obtained from the sensor located at a depth of 22.62 m were used because at this depth, the sensor is below the water level and is not affected by thermal interferences from the surface.
* Temperature data were recorded using specific software.