## 4 Results and Discussion
### Influence of Panel Tilt Angle on System Performances  
Based on the above analysis of the influence of tank installation height on thermal efficiency, the tank installation height is set as 0.7 m. As seen from Figure 8a, the bottom edge of the panel is in horizontal alignment with the upper edge of the window even though the tilt angle changes. Therefore, the values of A and \(H\) are respectively varied and unvaried. According to the latitude of Guangzhou, the range of panel tilt angle that helps to receive more solar radiation is set as 14\({}^{\circ}\) to 38\({}^{\circ}\). The resulting changes in shading coefficient can be seen from Figure 8b. Overall, the shading coefficient ranges from 0.797 to 0.828 when the tilt angle varies from 14\({}^{\circ}\)to 38\({}^{\circ}\), which means the reduction of total solar heat gains through window changes from 20.3% to 17.2% in a cooling season due to the exterior shading of the PV/T panel. The shading coefficient increases with the increase of tilt angle due to the decrease of overhanging length (\(A\)). From the angle of shading performance, the reduction of tilt angle helps to improve the shading performance of the PV/T panel. Figure 7: (a) Daily thermal efficiencies with various tank installation heights and panel tilt angles; (b) illustration for the variation of the vertical distance between the panel center and tank bottom. The thermal, electrical, and shading performances of the PV/T panel with various panel tilt angles were simulated. Figure 9 shows the annual useful thermal energy (\(Q_{u}\)), annual total solar radiation on panel (\(R_{T}\)), and annual electric energy output (\(E_{out}\)) with various tilt angles. Figure 10 shows the annual thermal efficiency (\(\eta_{th}\)), annual electrical efficiency (\(\eta_{e}\)), annual primary energy-saving efficiency (\(\eta_{f}\)), and annual auxiliary heat with various tilt angles. Although the annual total radiation on the panel reaches the maximum when the tilt angle ranges from 17\({}^{\circ}\) to 20\({}^{\circ}\), the useful thermal energy from the panel and annual thermal efficiency reach the maximum when the tilt angle ranges from 20\({}^{\circ}\) to 23\({}^{\circ}\). The annual electrical efficiency is not sensitive to the variation of the tilt angle, which is around 11.95%. The annual electric energy output reaches the maximum when the tilt angle equals 17\({}^{\circ}\) because the annual electric energy output mainly depends on the annual total radiation on the panel. The annual electric energy output changes slightly with the variation of the tilt angle and decreases by only 1.53 kWh when the tilt angle varies from 17\({}^{\circ}\) to 28\({}^{\circ}\). Figure 8: (**a**) Illustration for the variation of overhanging length; (**b**) shading coefficient vs. panel tilt angle. Figure 9: Annual useful thermal energy from panel (\(Q_{u}\)), annual total radiation on panel (\(R_{T}\)), and annual electric energy output (\(E_{out}\)) with various panel tilt angles. As a critical measurement index of solar energy utilization, the annual auxiliary heat reaches the minimum when the tilt angle equals \(28^{\circ}\). This can be explained by the variation characteristics of monthly useful thermal energy from the panel shown in Figure 11. From May to September, the monthly useful thermal energy increases with the decrement of the tilt angle. However, the monthly useful thermal energy decreases with the decrement of the tilt angle from November to January. There are no distinct differences among the amounts of monthly useful thermal energy with various tilt angles in the other months. Such variation characteristics of monthly useful thermal energy cause the situation that the smaller tilt angle leads to more monthly useful thermal energy in summer (the daily useful thermal energy is even surplus in some summer days) and less monthly useful thermal energy in winter (from November to January). Thus, the system with the smaller tilt angle needs more auxiliary heat in winter.