## 3 Methodology
### Data Collection and Pre-Processing  
A model was developed for predicting the coefficient of performance of a hybrid geothermal heat pump system by collecting data from the target building where the existing geothermal system was installed and operated and data from a test bed equipped with equipment that simulates the geothermal system. The coefficient of performance of the hybrid geothermal heat pump system is equivalent to predicting the coefficient of performance of a geothermal heat pump system. When operating a hybrid geothermal system, the circulating water heat exchanged in the ground undergoes additional heat exchange from an auxiliary heat source, which changes only the temperature entering the heat pump. Hence, a hybrid geothermal system coefficient of the performance prediction model can be developed using the data from the geothermal system. The actual geothermal system operation data were collected through the BAS system for the buildings in which the geothermal system was installed. Table 3 shows an overview of the building for geothermal system operation data collection. Additional data were collected from the test bed to develop a coefficient of a performance prediction model using the data of various temperature ranges that may occur during geothermal and hybrid geothermal systems operation. Figure 4 shows the configuration of the entire system used in the experiment, and the overview of the heat pump system is listed in Table 4.  
Figure 3: Process of the development of the performance prediction model. The test bed used a constant temperature tank connected to a heat pump to simulate the heat source and load side and the heat source side. It was used as the cooling and heating source and for maintaining the temperature through an air-cooled auxiliary heat source. The load and heat source sides were simulated by connecting a constant temperature tank to a heat pump to supply cooling and heating sources and maintain the temperature through an air-cooled auxiliary heat source. Table 5 outlines the experimental system, and Figure 5 shows the systems installed in the target building. In both systems, the factors affecting the performance coefficient of the geothermal heat pump, such as inlet and outlet temperature difference, flow rate, and power consumption, were collected for the performance evaluation in both systems, as well as the outside air temperature. Table 6 gives an overview of the operational data collection in both systems. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Classification** & **Cooling** & **Heating** \\ \hline Capacity (kW) & 190.61 & 178.93 \\ Power (W) & 38.03 & 44.52 \\ Flow rate (LPM) & 600 & 600 \\ COP & 5.01 & 4.02 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Overview of heat pump systems. Figure 4: Configuration of systems for operational data collection. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Category** & \multicolumn{2}{c}{**Contents**} \\ \hline \multirow{3}{*}{Building} & Use & Office \\  & Total floor area & 21,492 m\({}^{2}\) \\  & Size & B1/F9 \\ \hline \multirow{6}{*}{Geothermal system} & Type & Vertical closed-loop \\  & EA & 7 \\ \cline{1-1}  & Capacity (Cooling) & 105.6 kW \\ \cline{1-1}  & Capacity (Heating) & 101.4 kW \\ \cline{1-1}  & Number of borehole & 70 \\ \cline{1-1}  & Interval of borehole & 5 m \\ \hline \hline \multicolumn{2}{c}{Period of data collection} & 5 February 2014–31 August 2019 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Overview of buildings for geothermal system operation data collection. In the case of a building with a geothermal system installed, data monitored through BAS were collected at a three-hour cycle, and the data from January 1 to 31 December 2018, where data in various temperature ranges among the total collected data were analyzed, were used to predict the coefficient of performance.