## 4 Results and Discussion  
### Effect of Roof PV Forms on Natural Ventilation  
Figure 6 illustrates the wind velocity contours and vector diagrams of the substation under various scenarios. When the incoming wind faces the building, Case 1--with no PV system--is taken as the baseline. Particular attention is given to the main transformer room located on the windward side, including the airflow at its ventilation openings, within its interior, and across the rooftop space. The ventilation openings remain critical to facilitating natural ventilation in the main transformer room. Compared to theeward-facing Case 2, the airflow velocity at the inlets is higher in the windward-facing Case 1, and the airflow has a broader influence within the interior space. However, after entering the main transformer room, a significant portion of the airflow tends to converge along the walls with the overhead airflow within the building. When the PV system is installed horizontally (Case 3), the airflow at the ventilation openings shifts toward the center of the main transformer room, which is more favorable for internal ventilation. Simultaneously, the PV panels guide the airflow along the underside of the panels, aiding in the heat dissipation of the PV back surface. When the PV system is installed facing the wind (Cases 5 and 9), the inlet airflow also shifts toward the center of the transformer room. Case 9, which features an offset angle, performs better than Case 5, further demonstrating the influence of panel angle on ventilation. Additionally, the PV panels help channel the airflow beneath them, and the slope more effectively directs the air out. When the PV system is symmetrically installed (symmetrical pitched roof, Cases 13 and 15), the indoor stagnant zones are reduced, resulting in improved ventilation performance. In this configuration, the PV panels also achieve ventilation effects similar to those seen in windward-facing PV systems. In the case of leeward PV installations (Cases 6 and 10), the PV panels interact with airflow separation at the building's rooftop edge, guiding a portion of the high-speed airflow beneath the panels. However, this airflow does not enter the interior space. While this high-speed flow benefits the cooling of the PV back surface, it also increases the wind load on the panels, necessitating more robust structural support. The airflow at the ventilation openings remains largely unaffected, but a reduction in indoor stagnant zones is observed. When the incoming wind blows toward the building, Case 2--without a PV system--is taken as the reference. Particular attention is given to the leeward-side main transformer room, including its ventilation inlets, internal airflow, and rooftop space. On the windward side, airflow separates at the roof edge of the building, leading to wind speed-up. On the leeward side, a large wind shadow zone forms near the main transformer room, where wind speeds decrease and vortices occur. Most of the vortex airflow ascends along the exterior wall of the main transformer room toward the rooftop, while a smaller portion enters the room through the ventilation openings and merges with the vortex wake above the room. The wind velocity from the ventilation openings toward the transformer is appreciable; however, much of the interior space remains in a stagnant airflow zone. When the PV system is horizontally installed (Case 4), the PV panels disrupt the interaction between the vortex wake and the airflow entering through the ventilation openings, thus weakening the natural ventilation effect. Additionally, airflow beneath the PV panels is limited, which is also detrimental to the ventilation cooling of the PV back surface. When the PV system is installed in a windward orientation (Case 7 and Case 11) or symmetrically (Case 14 and Case 16), it does not cause significant disruption to the indoor airflow. The rooftop airflow is split by the PV panels and flows along their surfaces, which facilitates cooling of the PV backsheet.