## 4 Field Test Results and Discussions  
### Relative Heat Exchange Efficiency from TRTs  
Figure 9 shows temperature changes of the inlet and outlet fluid, and Figure 10 shows the average temperature of the inlet and outlet fluid versus the natural logarithm of time during the TRTs. In addition, Table 5 summarizes the calculated relative heat exchange efficiency per unit pipe length and the total pipe length of each energy slab. In order to compare the relative heat exchange performance of the energy slabs according to the pipe materials, the relative heat exchange efficiency per unit pipe length of the HDPE pipe and the STS pipe was compared for the floor-type and wall-type energy slab, respectively. In the case of the energy slab with the STS pipe, heat exchange performance is estimated to be about 66% higher than the energy slab with the HDPE pipe. This result suggests that if energy slabs are designed with the same pipe length, the STS pipe exhibits a higher heat exchange performance. As mentioned in Section 2.1, this improvement in performance seems attributable to the higher thermal conductivity (k = 16 W/mK) of the STS pipe compared to the thermal conductivity of the HDPE pipe (k = 0.4 W/mK). Therefore, when construction conditions (i.e., the material properties of ground formations and concrete slabs) are predetermined, selecting the heat exchange pipe with the higher thermal conductivity can be an effective way to enhance the thermal performance of energy slabs.  
\begin{table}
\begin{tabular}{c c} \hline
**Type** & **Specification** \\ \hline Bath size (dimension, W \(\times\) D \(\times\) H mm) & 350 \(\times\) 400 \(\times\) 300 mm \\ Bath capacity & 42 L \\ Power of heater/cooler & 4.0 kW/0.6 kW \\ Range of temperature capacity & \(-\)10 \({}^{\circ}\)C\(\sim\)98 \({}^{\circ}\)C \\ Temperature uniformity & \(\pm\)1 \({}^{\circ}\)C \\ Capacity of circulation pump & 20 L/min \\ Electric requirement & 220 VAC, 60 Hz \\ \hline \end{tabular}
\end{table}
Table 4: Specifications of constant-temperature water bath.  
Figure 9: _Cont._However, the temperature of ground formations and concrete slabs will rise or fall steeply due to excessive heat transfer in the case of a heat exchange pipe with an extremely high thermal conductivity. This phenomenon should lead to deterioration in the long-term thermal performance of energy slabs, which can reduce economic efficiency.  
The relative heat exchange efficiency per unit pipe length for the wall-type and floor-type energy slab was similar to each other when the heat exchange pipe was made of the same material (refer to Table 5). This means that there was no difference in the heat exchange performance between the two types of energy slabs in the TRT results. That is, in addition to a floor slab, a side wall slab can be used as a component of energy slabs to maximize the use of geothermal energy that can be generated from the underground space.  
Figure 10: _Cont._  
\begin{table}
\begin{tabular}{c c c c} \hline
**Type** & **Pipe Material** & **Pipe Length (m)** & _eff\({}_{pipe}\)_ \\ \hline Floor-type energy slab & HDPE & 85 & 7.57 \\ Wall-type energy slab & 58 & 7.26 \\ \hline Floor-type energy slab & 53 & 12.61 \\ Wall-type energy slab & 35 & 12.89 \\ \hline \end{tabular}
\end{table}
Table 5: Relative heat exchange efficiency per unit pipe length according to type of energy slab.  
Figure 9: Results of in-situ thermal response tests (TRIs) for energy slab (change in inlet and outlet temperature). (**a**) Floor-type energy slab (high-density polyethylene (HDPE)); (**b**) Floor-type energy slab (stainless steel (STS)); (**c**) Wall-type energy slab (HDPE); (**d**) Wall-type energy slab (STS).