## 1 Introduction
The thermosibhon cooling technique was combined with the floating PV systems, and this method enhanced the power of the PV system by 3.34%, compared to the floating system without the thermosibhon cooling, and by 7.86%, compared to the ground mounted PV system [16]. Experimental study on a heat pipe array with air cooling or water cooling on the condenser side indicated that the air cooling method produced an increase in efficiency of 2.6%, and the water cooling method produced a 3% increase in efficiency [17]. The experimental tests on the application of different types of phase change material for cooling the panels were conducted and concluded that up to a 10.26 \({}^{\circ}\)C temperature reduction can be obtained, resulting in an efficiency improvement of 3.73% [18]. Although a considerable cooling rate was achieved, when PCM is used, the system becomes bulky, with a large volume of PCM required, apart from the thermal conductivity issues within the material. Studies on the water cooling of PV panels include either submerging the panel in water or circulating water through the tubes attached to the backside of the panels. The experimental effects of submerging the panel in water concluded that the panel temperature could be maintained at 30 \({}^{\circ}\)C, increasing the relative efficiency by 20%, but with a reduction in the intensity of solar radiation falling on the collector [19]. The comparison of air- and water-based methods show that water-based cooling system performance is higher than the air cooling system [8]. Thermosibhon systems without phase change using water was tested using a copper sheet and tubing at the back of the panel with a water capacity of 80 L, and they reported an increase in the relative efficiency of 19% [20]. Experiments on forced convection cooling include forced air cooling by using specially constructed ducts on the back of the panel using a 3.6 W fan and reported an improvement in the overall efficiency of 0.6% [21]. The experimental results of water cooling by providing aluminum pipes on the backside of the panel reported an efficiency gain of 0.8% [22]. Evaporative cooling using water is a kind of phase change cooling that can be active [23; 24] or passive [25; 26]. Generally, evaporative cooling is very effective in regulating the temperature of the PV modules. However, active systems need extra power and more equipment, making them unattractive. While the passive systems mentioned above showed positive results, it was observed that the overall efficiency improvement was maximum in the case of thermosiphon method without phase change [20]. Additionally, the thermosiphon heat pipe method with the phase change tested on the concentrating collectors produced considerable cooling. Other methods showed inadequate heat transfer between the panel and the cooling fluid. The application of solid phase change material requires large volumes, due to the absence of re-circulation. In this study, a heat pipe thermosiphon (HPT), which is a closed-loop evaporative cooling system, is tested to maintain an appropriate range of operating temperature in a solar panel after its temperature reaches a particular level. An experimental investigation of using HPT with copper pipes was conducted under real environmental conditions. Three different quantities of the phase change liquid (PCL) were used to find the optimum quantity that achieves best results. The theoretical study involves a lumped capacitance thermal model, which is used to predict the temperature of the PV panels with and without HPT. The HPT cooling uses PCL, which evaporates by absorbing the heat produced by the panel once the panel temperature reaches its phase change temperature. Vapor rises up after evaporating at the heated region of the panel and condenses at the top section above the panel. It releases the heat to the surroundings through convection and radiation. Condensed liquid returns to the evaporation zone by gravity.