Abstarct: The surface temperature of photovoltaic panels increases with increasing solar radiation, which leads to reduction of electrical power of the panel and also its life. To solve this issue, various methods have been proposed, one of them is application of thermosyphon heat pipe. By connecting the thermosyphon heat pipe to the back of the panel as evaporator and also connecting another side of the pipe as condenser to a water tank, heat transfer from the panel to the water in the tank takes place and thus the panel surface temperature reduces. In this research, the effect of cross-sectional area of thermosyphon is evaluated theoretically and experimentally. A single heat pipe with circular cross-section and a single heat pipe with square cross-section are compared. In addition, the number of heat pipes effect on the performance of system is assessed. So, a single heat pipe and an array of two heat pipes with circular cross-section, are examined. In a new proposed model, to enhance the heat transfer, solar cells are mounted on an aluminum plate. Also, grooves are made on all three panels according to the cross section of the pipes along the length of the panel to increase the contact surface of the thermosyphon heat pipes with the aluminum plate. In the case of a single circular and square pipe, the location of the groove and the connection of the pipe in the center of the panel and for a double pipe, two grooves with a distance from each other in the center are considered. Three different values of filling ratio including 25, 45 and 65% are studied too. According to the experimental results, the best performance of system is obtained at a filling ratio of 45%. Due to its higher and more effective contact surface with the back of the panel, square cross section of heat pipe shows better performance than circular one. In this case, the amount of heat transferred to the water in tank for single square heat pipe was 26.45% higher than circular single heat pipe. ‌In addition, application of two heat pipes which are connected to the panel compared to application of single circular heat pipe leads to 47.03% more heat transfer. In the case of electrical performance, by application of single circular and square heat pipe at filling ratio of 45%, 2.51 and 3.85% more output power are recorded than conventional panel, respectively. In this dissertation, the modeling of system is done, too. The proposed mathematical model is validated by experimental results. The percentage of instantaneous error for reservoir water temperature in the case of single circular, square and double circular pipes are obtained to be around 3.75, 11.54 and 4.41, respectively. Also, the average error in temperature differences, which is an exxpression of error between modeling and experimental results, is around 27.62%.