## 4 Results and Discussion
### Effect of Roof PV Forms on Natural Ventilation
Among these, the airflow on the backside of the PV panels in Case 11 is better than in Case 7, and in Case 16, it is better than in Case 14, further highlighting the influence of installation angle on PV system ventilation performance. When the PV systemis installed in a leeward configuration (Case 8 and Case 12), the PV panels contribute to the airflow separation occurring at the building's rooftop edge and direct part of the high-speed airflow into the main transformer room. However, this high-speed airflow conflicts with the natural airflow entering through the ventilation openings, thereby weakening the room's ventilation effectiveness and expanding stagnant airflow zones. Although this redirected airflow benefits PV backsheet cooling, it also increases the wind load on the PV panels, imposing stricter structural requirements for panel mounting. In particular, the wind speed-up effect in Case 11 is more pronounced than in Case 7, indicating that when PV panels are installed leeward with a tilt, the angle significantly affects the wind environment above the building. To further quantify the impact of different PV system configurations on ventilation in the main transformer room, three vent area (vent areas A, B, and C, as shown in Figure 7) were defined at critical locations where the room exchanges air with the external environment--specifically at the rooftop and bottom space. In the post-processing  
Figure 6: Contours and vector diagrams for different cases. stage, the area-weighted average method was used to calculate the velocity. Since areas A and B are identical in size and share the same ventilation conditions, their values were combined to obtain a net value representing the bottom ventilation performance. Figure 8 shows detailed data for all cases. The ventilation performance of Case 1 (transformer orientation: windward side) and Case 2 (transformer orientation: leeward side) is used to evaluate the baseline heat dissipation level of the main transformer room without PV systems. This serves as the reference for comparing the impact of various PV installation strategies on thermal performance. The model adopted in this study is based on a standard construction scheme that has been in practical use for nearly two decades, and its effectiveness in meeting heat dissipation requirements under typical wind conditions has been well validated. Using this as a benchmark, other cases are compared to assess whether the proposed PV installation methods meet the heat dissipation demands of the electrical system. Case 3 and Case 4 are used as reference cases. Both use horizontally mounted PV systems. These are used to compare and assess the impact of PV installation on ventilation performance. Detailed data can be found in Appendix B. First, the impact of PV system installation on ventilation in the main transformer room is evaluated. Compared to the initial condition without PV systems, the comparison between Case 1 and Case 3, as well as between Case 2 and Case 4, shows that horizontal PV installation reduces airflow velocity in the ventilation zones. Specifically, in the windward scenario, the wind speed at the bottom ventilation area is reduced by 13.2%, while in the leeward scenario, it is reduced by 11.8%. The ventilation speed in the overhead space remains low in both conditions and is largely unaffected. This indicates that the commonly used horizontal PV installation method has a negative impact on substation ventilation. Next, this study evaluates whether the proposed PV installation methods can mitigate the ventilation rate losses caused by PV system placement. Under the windward condition of the main transformer room, the PV layout strategies proposed in this study (Case 5, Case 6, Case 9, Case 10, Case 13, and Case 15) all show significant improvements in ventilation performance compared to Case 3. The increases are 12.6%, 11.3%, 11.9%, 13.2%, 8.6%, and 14.6%, respectively. Compared to Case 1, all of these layout strategies demonstrate good ventilation performance.