## 3 Results and Discussion
Consequently, the cooling demand is increased. Figure 12c shows the total annual energy demand including heating and cooling for all the selected double glazing. The PC-16 mm material is more energy efficient than the double-glazed material. Greenhouse orientation has a significant effect on passive energy saving techniques. The effect of different greenhouse orientations was studied with the physical operating conditions described in Table 5. The total annual energy demand, including heating and cooling, was estimated for each  
Figure 12: Annual energy demand of greenhouse with double-glazing (**a**) Heating (**b**) Cooling, (**c**) Total. orientation and the results are shown in Figure 13. The results indicate that the E-W orientation has the smallest annual energy requirement. This is due to the fact that, in winter, the E-W orientation receives more solar radiation than the other orientations, and the reverse happens in summer. Figure 14 shows the average daily solar energy gain inside the greenhouse during January (winter) and June (summer). The result indicates that, in January, the average daily solar gain of the E-W (0\({}^{\circ}\)) orientation was 11%, 14%, 15%, 12%, and 7% higher than the 15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), 75\({}^{\circ}\), and 90\({}^{\circ}\) (N-S) orientations, respectively. Moreover, during June, the E-W (0\({}^{\circ}\)) orientation received 7%, 11%, 13%, 15%, and 15% less average solar gain than that of the 15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), 60\({}^{\circ}\), 75\({}^{\circ}\), and 90\({}^{\circ}\) (N-S) orientations, respectively. A similar study conducted for energy efficient design of a multi-span greenhouse also confirmed this trend [1]. Natural ventilation is widely used to decrease the internal temperature of greenhouses, especially in the summer. In this study, we evaluated the effect of natural ventilation on the cooling demand of the multi-span greenhouse. We estimated a monthly cooling demand of a fully closed and naturally  
Figure 14: Annual energy demand for the greenhouse with different orientations. Figure 13: Annual energy demand for the greenhouse with different orientations. ventilated greenhouse and the results are shown in Figure 15. Again, all physical and operating conditions are same ones presented in Figure 9:0 The results indicate a 50% reduction in cooling demand during the whole summer season using natural ventilation combined with cooling. Figure 16 shows the internal temperature of the greenhouse with natural ventilation and a fully-closed condition. It can be seen that in the fully-closed condition with no ventilation, the greenhouse's internal temperature increases to 60 \({}^{\circ}\)C. However, with natural ventilation, it reduces to 40 \({}^{\circ}\)C, which causes a reduction in the cooling demand in the natural ventilation condition. The TRNSYS model successfully described the natural ventilation effect combined with a greenhouse thermal model. Figure 9:1 Comparison of greenhouse internal temperature with natural ventilation and fully closed conditions. Figure 9:2 Comparison of monthly cooling demand of the greenhouse with natural ventilation and fully closed conditions. In the northern hemisphere, the sun stays at the south side and north wall of the greenhouse and contributes less to the solar heat gain inside the greenhouse. Other studies reported that the use of an opaque north wall can reduce 25-30% of the heating requirement in winter when compared to using a transparent north wall during the daytime. Therefore, it is recommended that insulation should be used on the north wall of the greenhouse to reduce heat loss, especially in colder regions during winter [4; 37]. Commonly, multi-layered thermal screens are used for north wall insulation in South Korea.