## 4 Investigations and Discussion
### Characteristics of Effective Thermal Conductivity of SCW System with Single- and Two-Well Types  
Figure 11 shows the results of thermal performance according to the bleeding rate of groundwater using single-well and two-well SCW systems as in References [38; 39; 40; 54; 55; 56]. Figure 11a,b show the thermal performance of a single-well SCW system, as shown in Figure 5c. A study on the effective thermal conductivity characteristics was carried out according to the bleeding effect in the single-well SCW system. Figure 11c,d show the thermal performance of a two-well SCW system, as shown in Figure 5d. As shown in the schematic of Figure 5d, a study was conducted to optimize heat transfer characteristics through energy circulation caused by groundwater flow by injecting the groundwater bleeding amount from one intake hole. Therefore, thermal performance improvement through effective thermal conductivity was analyzed through the thermal performance evaluation of the SCW underground heat exchanger. Figure 10: Resistance value at the lower and upper locations of the return pipe. From the results of Figure 11a,b, obtained through the average inlet/outlet temperature according to In (t) using the line-source method, the k-values were 4.88 and 2.94, respectively. The effective thermal conductivity values shown in Table 7 were 3.45 and 5.59 W/(m\(\cdot\)K). From this result, the improvement of thermal performance was improved by 62% when bleeding rate was performed. In the case of single-well SCW, the heat transfer characteristics were improved by minimizing the energy change of the groundwater because of the amount of water flowing from the underground hole into the groundwater. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Type**} & \multirow{2}{*}{**Unit**} & \multicolumn{2}{c}{**Single SCW**} & \multicolumn{2}{c}{**Single SCW**} & \multicolumn{2}{c}{**Two-Well SCW**} & \multicolumn{2}{c}{**Two-Well SCW**} \\  & & **(No bleeding)** & **(10\% Bleeding)** & **(No bleeding)** & **(25\% Bleeding)** \\ \hline Initial temperature & \({}^{\circ}\)C & 16.13 & 16.13 & 17.25 & 15.64 \\ Average temperature & \({}^{\circ}\)C & 34.45 & 32.44 & 40.92 & 30.91 \\ Heat injection & kW & 40 & 40 & 78 & 81 \\ Slope (K) & - & 2.937 & 2.937 & 7.525 & 1.88 \\ Effective thermal conductivity & W/m\(\cdot\)K & 3.45 & 5.59 & 2.4 & 9.59 \\ Ratio of bleeding to no bleeding & \% & - & 162 & - & 400 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison on the measuring of thermal conductivity in various bleeding conditions. Figure 11: Linearization of the average temperature of the single- and two-well type SCW systems: (**a**) single-well type (no bleeding); (**b**) single-well type (10% bleeding); (**c**) two-well type (no bleeding); (**d**) two-well type (25% bleeding). Table 7 also shows the line-source method results of Figure 11c,d using two-well SCW underground heat exchangers. Finally, the thermal performance of the system improved by about four times compared to the bleeding rate. This improvement of the effective thermal conductivity was the result of the groundwater being well used as the circulating water. Figure 11d shows the characteristic that the value of k (i.e., the exact slope) changed using the IIT (initial ignoring time) method. It was confirmed that the experiments need to be carried out through a longer-term performance evaluation relative to the current measurement method, as this affects the final effective thermal conductivity. As shown in Figure 11a,b, the k value and the effective heat transfer value were almost constant. However, in the two-well SCW of Figure 11c,d, the value of k decreased with time (Figure 12). The reason for this is because the groundwater temperature uses energy without the change of the Bleeding water injected from the intake hole to the return hole. It was judged that the two-well SCW system can maximize the heat consumption in a narrow area by using a geothermal heat exchanger.