## 2 Construction of the Energy Slab in a Test Bed  
### Configuration of the Energy Slabs  
As the thermal conductivity of surrounding media increases, the thermal performance of GHEXs increases. Therefore, increasing the thermal conductivity of the surrounding media is one of the effective ways to enhance the thermal performance of GHEXs. However, it is challenging to significantly increase the thermal conductivity of ground formations and concrete slabs because they are predetermined according to the construction specification. Therefore, enhancing the thermal conductivity of heat exchange pipes is considered in this study. Field-scale energy slabs were constructed with the heat exchange pipes made of two different materials, i.e., HDPE and STS, to evaluate the effect of the pipe material on the thermal performance of energy slabs. Figure 1 shows the configuration of each pipe type. The HDPE pipe was spirally installed in the energy slab because it is difficult to make a small radius of curvature. Meanwhile, the STS pipe was arranged in a row because the STS pipe is pre-manufactured in a straight configuration. Then, the distance between adjacent pipes (i.e., 300 mm for the HDPE pipe and 500 mm for the STS pipe) was determined at the design stage so that the maximum length of pipe could be installed with consideration of the field conditions in the test bed. Energy slabs can be constructed not only on the floor, but also on the side wall in a basement, which can utilize underground space more effectively. Therefore, both the floor-type and wall-type energy slabs were constructed in the test bed to compare the thermal performance of each type of energy slabs. Figure 2 shows a schematic description of the floor-type and wall-type energy slab in the test bed, and Table 1 summarizes the details of constructed energy slabs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** & **Material** & \begin{tabular}{c} **Thermal Conductivity** \\ **(W/mK)** \\ \end{tabular} & **Length (m)** & **Pipe Diameter (mm)** \\ \hline Floor-type energy slab & \begin{tabular}{c} High density \\ polyethylene (HDPE) \\ \end{tabular} & 0.4 & \begin{tabular}{c} 85 \\ 58 \\ \end{tabular} & 40 \\ \hline Floor-type energy slab & \
\begin{tabular}{c} Stainless steel (STS) \\ \end{tabular} & 16 &
\begin{tabular}{c} 53 \\ 35 \\ \end{tabular} & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Length and thermal conductivity of each heat exchange pipe. Figure 1: Configurations of each pipe type. (**a**) high-density polyethylene (HDPE) pipe; (**b**) stainless steel (STS) pipe. Figure 3 shows a sectional view of the constructed energy slab. Generally, as an energy slab is in operation, the temperature of concrete slabs continuously changes. Because the concrete slab is in contact with the ambient air in the underground space, temperature changes in the concrete slab can degrade the indoor heating, ventilation and air-conditioning (HVAC) efficiency. In contrast, the thermal performance of heat exchangers is reduced by the influence of the indoor air temperature. Conceptually, during heating/cooling operations, indoor air temperature of the underground space, which is maintained at the target heating/cooling temperature, is different from the working-fluid temperature within the heat exchange pipes. Heat exchange between the heat exchanger and ambient air can take place when no thermal insulation is equipped over the heat exchanger, which may reduce the thermal performance of heat exchangers. Therefore, a thermal insulation layer between the heat exchanger and ambient thermal environment should be arranged, to minimize such mutual influence. In the test bed, a phenol foam (PF) board with the thermal conductivity of 0.018 W/mK was adopted as a thermal insulation material, and exclusively installed in the floor-type energy slab. On the other hand, no thermal insulation layer was inserted in the wall-type energy slab. Temperature sensors were installed to monitor the temperature change of concrete slabs during the operation of energy slabs.