## 4 Field Test Results and Discussions
### Evaluation of Thermal Performance from TPT  
A series of TPTs were conducted for the four types of energy slabs in the heating operation condition. Table 6 summarizes the experimental conditions. The flow rate of a working fluid was determined to make the Reynolds number (Equation (4)) greater than 4000, which can induce turbulence flow inside the pipe. Note that the temperature at the inlet was maintained at 5 \({}^{\circ}\)C for the heating mode in the energy slabs. \[\mathrm{Re}=\frac{\rho vD}{\mu}>4000 \tag{4}\]  
Figures 11-14 show the results of the TPTs. In order to evaluate the effect of a thermal insulation layer, the different types of energy slabs that encases the same pipe materials (i.e., HDPE and STS pipe) were compared to each other. First, in the case of the energy slab with the HDPE pipe, the heat exchange amount was measured at almost zero during the whole test when the energy slab was installed without a thermal insulation layer as shown in Figure 11. In other words, the heat transfer between the working fluid and the surrounding medium could not occur due to the significant influence of the ambient air temperature. It can be found in Figure 15 that the temperature at B in the concrete slab (refer to Figure 3) showed a similar response to the ambient air temperature. As a result, the outlet fluid temperature monitored during the TPT also varied with the ambient air temperature variation as shown in Figure 11. \begin{table}
\begin{tabular}{c c} \hline \hline
**Type** & **Condition** \\ \hline Operation type & 8 h operation–16 h pause (intermittent operation) \\ Flow rate & 5 L/min \\ Inlet temperature & 5 \({}^{\circ}\)C (heating condition) \\ Sample interval & 5 min \\ Total test duration & 7 days \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Test conditions of in-situ thermal performance tests (TPTs). Figure 10: Results of in-situ thermal response tests (TRTs) for energy slab (average temperature changes of inlet and outlet versus natural logarithm of time). (**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). Figure 11: Thermal performance test (TPT) results of the wall-type energy slab with the high-density polyethylene (HDPE) pipe (without thermally insulation). (**a**) Inlet, outlet temperature and heat exchange amount; (**b**) Outlet and ambient air temperature. Figure 12: _Cont._  
On the contrary, when a thermal insulation layer is installed, a relatively constant temperature was observed at B' in the concrete slab (refer to Figure 3) during the operation of the heat exchanger as shown in Figure 15b. While the temperature measured at A' in the concrete slab showed a similar response to the ambient air temperature (refer to Figures 3 and 15). Consequently, the existence of a thermal insulation layer induced a measurable heat exchange amount during the entire test as shown in Figure 12.  
When the STS pipe was installed without a thermal insulation layer in the energy slab, sufficient heat exchange occurred at the early stage of the test, because the air temperature was sufficiently high enough to induce heat exchange at this stage (refer to Figure 13b). However, as the air temperature decreased, a lower heat exchange amount was observed due to the influence of air temperature (refer to Figure 13b). In general, the heat exchange amount of GHEXs reduces as time goes by because the heating (or cooling) load is continuously accumulated in the ground. As shown in Figure 14, this tendency to heat exchange was observed in the energy slabs with a thermal insulation layer, which means that the thermal insulation layer can effectively isolate the energy slab from ambient air temperature during the tests. Figure 12: Thermal performance test (TPT) results of the floor-type energy slab with high-density polyethylene (HDPE) pipe (with thermally insulation).