## 4 Investigations and Discussion  
### Thermal Conductivity Characteristics of Various Rocks in a Closed borehole Heat Exchanger System  
Figure 6 shows the average temperature and the difference of inlet/outlet temperatures over time for alluvial deposit, gneiss, and granite rocks using a ground heat exchanger system with a borehole depth of 200 m using a closed loop borehole heat exchanger system [3; 6; 13; 41; 49; 53]. The initial groundwater temperature from the three types of rock was approximately 15-16 \({}^{\circ}\)C, with an average temperature distribution of approximately 28-35 \({}^{\circ}\)C during the test period. The average temperatures obtained from the three types of rock are shown in Figure 7, linearized according to \(\ln\) (t). The slope of the graph is linearized by the line-source method using means of k. The effective heat transfer coefficient can be obtained by substituting k for Equation (6). If heat quantity (Q), borehole depth (H), and slope (k) are substituted into Equation (1), effective heat transfer and slope values can be obtained as shown in Table 5. From Equation (6), it can be seen that k and \(\lambda\) are in a trade-off relationship with each other. The thermal performance evaluation results of the ground heat exchanger for the three types of rock show that the effective thermal conductivity ranges from 2.5 to 3.2 W/(m\(\cdot\)K). Based on the results of this study, it is possible to grasp the thermal conductivity of daughters in the case of the same borehole depth according to the characteristics of the rock. In addition, the influence of groundwater is considered to be an influential factor on the effective thermal conductivity. In the future, the effect of groundwater flow on the effective thermal conductivity of underground heat exchangers will be studied. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Actual Flow (LPM)** & **Calibration Flow (LPM)** & **Error (\%)** \\ \hline
3.316 & 3.292 & 0.73 \\
15.87 & 16.032 & 1.01 \\
100.9 & 102.99 & 2.03 \\
350.62 & 355.41 & 1.35 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Results from the flowmeter calibration. Figure 6: Average temperature distribution of various of rocks. Characteristics of the Effective Thermal Conductivity of SCW System at Different Locations of Return Pipe  
Figures 8 and 9 show the evaluation results of thermal performance by temperature for a ground heat exchanger with the return pipe positioned as shown in Figure 5b, using an SCW underground heat exchanger with a borehole depth of 400 m, as shown in References [31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. This study analyzed the characteristics of this in accordance with the effective thermal conductivity and thermal resistance as a study on the heat transfer characteristics according to a position above the submerged pump [52; 56; 57; 60]. Through the research results of [PERSON] et al. [4; 11; 41], it was found that the effective thermal conductivity and the thermal resistance were improved according to the effect of the position of the pump. The k-values obtained by the line-source method for the lower (Figure 8) and upper return pipes (Figure 9) were 4.26 and 4.56, respectively. Using Equation (6), the effective thermal conductivities of the lower (Figure 8) and upper return pipes (Figure 9) were 3.86 and 3.54 W/(m \(\cdot\) K), respectively, using a borehole insertion depth of 400 m with injected calories of 48 kW. The results are shown in Table 6 and confirm that the performance improved by about 9% depending on the return pipe position of the circulating water. From the result of Figures 8 and 9, the effective thermal conductivity increased at the lower location of the return pipe more than at the upper location. Through these results, it was judged that the technology can improve thermal performance. This can be important research data to improve the heat transfer characteristics in the same ground hole by considering the location of the submerged pump.