## 4 Investigations and Discussion
### Thermal Conductivity Characteristics of Various Rocks in a Closed borehole Heat Exchanger System
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Unit** & **Alluvium \#1** & **Alluvium \#2** & **Geniss \#1** & **Granite \#1** & **Granite \#2** \\ \hline Initial temperature & \({}^{\circ}\)C & 15.2 & 15.4 & 16.2 & 16 & 14.5 & 16.3 \\ Average temperature & \({}^{\circ}\)C & 28.5 & 31 & 31 & 33.2 & 28.4 & 30.1 \\ Heat injection & kW & 14.5 & 13.9 & 13.8 & 14 & 14.1 & 14.0 \\ Slope (K) & - & 2.151 & 2.175 & 1.584 & 2.094 & 1.945 & 2.094 \\ Effective thermal conductivity & W/(m\(\cdot\)K) & 2.614 & 2.663 & 3.213 & 2.648 & 2.813 & 2.518 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Evaluation of effective thermal conductivity and experimental conditions at borehole depth of 200 m in various rocks. Figure 7: Linearization of line-source method for various rock distributions. Figure 10 shows the thermal resistance in the SCW system [6; 53; 54]. [PERSON] et al. 's research [6] suggested two main objectives: (i) to determine the variability of the predicted ground thermal conductivity for the same evaluated ground, and (ii) to determine the influence of the type of growth used in the estimated borehole thermal resistance. As shown in Reference [6], the effective \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type** & **Unit** & \
\begin{tabular}{c} **Single SCW (Lower** \\ **Location of Pump)** \\ \end{tabular} &
\begin{tabular}{c} **Single SCW (Upper Location of Pump)** \\ \end{tabular} \\ \hline Initial temperature & \({}^{\circ}\)C & 15.27 & 16.13 \\ \hline Average temperature & \({}^{\circ}\)C & 34.45 & 32.44 \\ Heat injection & kW & 48 & 48 \\ Slope (K) & - & 4.146 & 4.545 \\ Effective thermal conductivity & W/m-K & 3.86 & 3.54 \\ \hline Ratio of bleeding rate to no bleeding & \% & 109 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Results of effective conductivity calculation. Figure 8: Linearization of the average temperature at the location of the lower return pipe. Figure 9: Linearization of the average temperature at the location of the upper return pipe. thermal conductivity of this system was obtained by substituting the values into Equation (6). Finally, the thermal resistance value was obtained, as shown in Figure 11. After 60 min at the initial condition, the thermal resistance value showed sudden and repetitive falling and rising before converging to a constant value. Despite the short measurement time of 12 h, the overall thermal resistance value converged to a constant. At the lower return pipe, the thermal resistance value also generally converged to a constant value. The average thermal resistance values obtained using Equation (7) at the lower return pipe and at the upper return pipe were approximately 0.0118 K/(W/m) and 0.0053 K/(W/m), respectively. It can be observed that the thermal resistance value at the lower return pipe was higher than that at the upper return pipe by more than 44.91%. Considering these results, it was found that the thermal resistance value acts as an important variable among the design factors in the standing column well-type underground heat exchanger. In this study, the effective thermal conductivity and thermal resistance values of the standing column well-type underground heat exchanger were measured by using a thermal response test device for the case where the return pipe is located at the bottom and the return pipe is located at the top. The effective thermal conductivity and thermal resistance values of the case where the return pipe is located at the bottom were increased by about 6.56% and 44.91%, respectively, compared to the case where the return pipe was located at the top.