## 2 Experimental Details
### Design and Operation Details  
The dual-fluid heat exchanger for the cooling of the PV module was constructed using serpentine shaped copper pipes as the liquid fluid carrier and a rectangular duct made of PVC for an air carrier. The inner diameter of the copper pipe was 8 mm and the height and width of the air passage were 50 mm and 979 mm, respectively. The absorber sheet with 1 mm thickness was attached at the rear surface of the PV module by means of adhesive and then the copper tube was glued to the absorber sheet. A mono-crystalline PV module with a surface area of 1619 \(\times\) 979 mm was covered with tampered glass having a thickness of 4 mm. The specifications of the PV module is given in Table 1. An air duct was arranged directly below the copper pipes such that both fluids exchanged heat with each other without any physical interaction. In addition, a set of baffles was introduced inside the air passage to enhance the heat exchange across the channel's surfaces.  
The PV module and the two fluids-based heat exchanger were assembled to form a dual-fluid PV/T system that was installed at the College of Engineering, Kongju National University, Cheonan campus, as shown in Figure 1. The coordinates of Cheonan city are 36.815\({}^{\circ}\) N, 127.1139\({}^{\circ}\) E. Experimental data were collected in four different phases. The starting experiments used a reference PV module without any cooling medium; then in the 2 nd and 3 rd phases solely air and water, respectively, were tested for PV cooling. Lastly, the performance of the dual-fluid (water plus air) heat exchanger was studied. The thermal and electrical performance of the proposed PV/T system was evaluated across different flow regimes. The tested flow rate ranges for water and air were 0 to 0.032 kg/s and 0 to 0.05 kg/s, respectively. Considering the freezing-cold climate of Korea in winter, an antifreeze  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Cell Type Silicon** & **Mono-Crystalline** \\ \hline Module size & 1619 \(\times\) 979 mm \\ P\({}_{\text{max}}\) & 260 W \\ V\({}_{\text{max}}\) & 31.6 V \\ I\({}_{\text{max}}\) & 8.23 A \\ V\({}_{\text{oc}}\) & 38.1 V \\ I\({}_{\text{sc}}\) & 9.27 A \\ Aperture ratio & 48\% (8 mm holes) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications of the photovoltaic (PV) module.  
liquid was mixed with water to lower the freezing point of the mixture. A mixture of water and antifreeze was used as a circulating fluid which was stored in an auxiliary tank as shown in Figure 2. Temperature variations across different components of the PV/T system were recorded by means of thermocouples. Quantities and employed positions of the temperature sensors were as follows: two PT100 \(\Omega\) temperature sensors were positioned each at the inlet and outlet of the water component; six thermocouples were mounted each at the inlet and outlet of the air cooling unit; four thermocouples were attached each at the top surface of the PV module; and three thermocouples were installed both at the absorber sheet and the tubes carrying water. Daily current and voltage outputs from the PV module were monitored by a PV Module I-V measurement system (TNE TECH CO. LTD, Yongin, Korea) and stored in a computer via a data acquisition unit (Agilent 34970A, Santa Clara, CA, USA). Solar radiation was measured with the help of a first class pyranometer mounted parallel to the PV module. Digital flow meters were used to measure air and water flow rates. Wind speed, ambient temperature, and humidity were measured with a weather station installed near the test rig. Data collection of the aforementioned parameters involves instrumentation, and therefore, prior to experiments, all measuring instruments were calibrated properly following standard procedures. The uncertainty related to measuring equipment is given in Table 2.  
Figure 1: (**a**) Experiment setup and (**b**) flow schemes.