## 2 Experimental Study  
Figure 1 shows the PV/HPT cooling experimental setup. The PV/HPT system comprises a solar PV panel mounted on inclined metallic frame work with the heat pipe tubes slightly flattened and attached to the bottom of the panel (Figure 1a,b). Acetone is the used as a PCL, due to its desirable phase change temperature of 56 \({}^{\circ}\)C [27]. Heat is rejected to the surrounding in the extended portion of the heat pipe where condensation of the working fluid (WF) takes place. The panel attached with HPT was filled with the acetone after completely evacuating the air from it. The HPT tube is attached at the back of the panel. Another PV panel without a heat pipe is used as a reference for the comparison. The heat transfers by conduction through the layers of the PV panel and then into the HPT tube, which contains the WF. As the temperature rises and reaches the phase change temperature, the evaporation of WF takes place, and the vapor rises up along the tube to the condenser region. The condenser region condenses the vapors on its walls by dissipating heat to the surroundings, which is at a lower temperature than the condensing temperature of WF. The heat is transferred to the surroundings by convection and radiation from the HPT. The condensed liquid flows  
Figure 1: (**a**) Experimental setup, (**b**) HPT’s arrangement. down to the evaporating zone, and the cycle is continued, thereby maintaining the desired temperature of the panel. The WF selection is based on the temperature of the panel to be maintained. The following instruments were used during the experimentation process:  
1. A pyranometer to measure the global solar radiation. 2. Data acquisition to record the data and a personal computer. 3. K-type thermocouples to measure the temperatures at different location. The panels were mounted on steel frames at a height of 1.75 m from the floor. The floor was made of red clay tiles. The panels were placed at an angle of 24\({}^{\circ}\) and were oriented towards the south in Riyadh, KSA. The diameter and thickness of the copper HPT were 0.025 m and 0.003 m, respectively. The area of the contact of HPT on the panel bottom surface was 0.0109 m\({}^{2}\). The length of the HPT was 1.5 m.  
Four K-type thermocouples were fixed on each panel at four different locations, with two points on the front side and the other two on the back side. Another thermocouple was also used to record the ambient temperature during the experimentation. The pyranometer was also fixed in a tilted position on the frame holding the panels. The thermocouples and pyranometer were connected to the data acquisition system, which was, in turn, connected to a computer with Daq plus software installed on it. The specifications of the thermocouple used is given below:  
Material was (CHROMEGA(\({}^{\circledR}\)-ALOMEGA(\({}^{\circledR}\)), maximum temperature is 175 \({}^{\circ}\)C (350 \({}^{\circ}\)F) continuous, minimum temperature is \(-60\)\({}^{\circ}\)C (\(-75\)\({}^{\circ}\)F) continuous, and dimensions were 25 L \(\times\) 19 W \(\times\) 0.3 mm (1 \(\times\) 0.8 \(\times\) 0.01 inch\"). The calculated uncertainty of the thermocouple at level of confidence 95% was \(\pm\)0.0153 \({}^{\circ}\)C.  
The test was conducted during the summer conditions, from 9 a.m. to about 3 p.m. The temperatures of the reference panel and the panel with the thermosiphon were measured simultaneously. Specifications of HPT and WF  
Polycrystalline PV panels from BP SOLAR were used, with 350 W\({}_{\p}\), and the area was 0.839 m \(\times\) 0.537 m. The dimensions and material properties of the panel are given in Table 1.  
The reference panel temperature was measured using thermocouple R1-R5, and the average value was used. The cooled PV panels was fitted with one HPT's, which was secured to the panel bottom, along with thermal conducting paste to ensure good heat transfer (Figure 2b). The HPTs were slightly flattened at the area of contact with the PV panel.