Evaluating the Ventilation Performance of Single-Span Plastic Greenhouses with Continuous Screened Side Openings

Abstract

Natural ventilation is the most cost-effective environmental control method for protected horticulture. To overcome the issues associated with high temperatures in greenhouses, analyzing their ventilation characteristics and maximizing their natural ventilation performance are essential. Therefore, in this study, the natural ventilation performance of arched single-span plastic greenhouses with screened side openings was empirically investigated. Three identical single-span plastic greenhouses were used in this study, each with different side-opening heights. Temperature and wind-speed data were collected, and the ventilation-volume flow rate was calculated considering both buoyancy and wind forces. The natural ventilation performance of the greenhouses was strongly and positively correlated with the ventilation-area ratio and outdoor wind speed. The ventilation rate increased linearly with an increase in the ventilation-area ratio and high outdoor wind speeds. However, the association between ventilation performance and indoor–outdoor temperature differences was not strong. When the ratios of ventilation areas in the greenhouse were changed to 0.08, 0.19, and 0.29, the average indoor–outdoor temperature differences were 14.0, 10.1, and 7.7 °C, respectively, and the ventilation rates were 0.0081, 0.0196, and 0.0315 m³ s⁻¹ m⁻², respectively. The proportion of wind in the total ventilation performance was high with a low ratio of ventilation openings and high outdoor wind speeds. However, the proportion of the buoyancy was high, with a high ratio of ventilation openings and a large indoor–outdoor temperature difference. Overall, this study provides foundational insights for optimizing the design and evaluation of ventilation openings in greenhouses, considering the outdoor wind speed.

1. Introduction

Recently, the widespread implementation of smart protected horticultural agriculture in South Korea has significantly improved the quality of greenhouse crop cultivation, warranting steps aimed at ensuring the year-round productivity of these systems. However, owing to the negative impact of the cooling load on the agricultural yields of these greenhouses, the annual utilization rate of these systems (i.e., despite their inherent effectiveness) has remained low. Therefore, to sustainably grow high-quality crops in greenhouses during summer, it is necessary to prepare and create appropriate measures to mitigate the issues associated with high temperatures. High temperatures in greenhouses can be overcome as follows: ventilation, shading, evaporative cooling, and cooling using heat pumps. Overall, ventilation performance is a fundamental design factor in the introduction of such a system [1,2].

Ventilation is the replacement of the air inside a greenhouse with outside air, and it plays an essential role in stabilizing crop production and improving the microclimate in a greenhouse [3]. When crops are grown in a closed greenhouse, the air temperature inside the greenhouse is higher than the maximum set temperature. This causes high-temperature damage and poor humidity, making it impossible to grow crops [4,5]. Therefore, greenhouse farmers have installed various ventilation devices to control the thermal environment in greenhouses. Ventilation is broadly categorized into forced ventilation (which involves the use of mechanical devices such as fans) and natural ventilation (which involves the use of side and roof openings), with natural ventilation being the most economical and sustainable option [6,7,8]. Research on ventilation in greenhouses has focused on natural ventilation over the past 50 years. Al-Helal et al. [9] presented a thermal model that can predict the ventilation rate in a greenhouse using the fundamental heat balance of the greenhouse. Boulard and Baile [10] theorized the mechanisms of ventilation in greenhouses. They proposed an equation to calculate the volume flow rate using Bernoulli’s equation. Kim et al. [11] reported that the indoor–outdoor temperature difference decreased from 14.0 °C to 7.1 °C when the side window opening height was increased from 0.6 m to 1.2 m in a naturally ventilated greenhouse. Liu et al. [12] reported that greenhouses combined with side openings and roof openings showed advantages in indoor air-temperature decrease than those equipped with roof openings only. Chu and Lan [13] reported that the ventilation performance of greenhouses can be improved when the ventilation openings are faced toward the outdoor wind direction. Ganguly and Ghosh [14] experimentally investigated that indoor air temperatures were decreased at a higher wind speed. However, there is no technology that can accurately predict and control the natural ventilation performance of greenhouses. Until now, studies have been mainly conducted on the theoretical analysis of volume flow rate.

Most greenhouses in South Korea are arched single-span plastic greenhouses. In 2022, Korea’s facility cultivation area occupied 52,808 ha, with single-span plastic greenhouses accounting for 83.8% (44,268 ha) of it [15]. It is difficult to install roof openings in arched single-span plastic greenhouses; therefore, most of these are equipped with roll-up side openings [16,17]. Therefore, maximizing, predicting, and evaluating the natural ventilation performance of greenhouses that rely on side-opening ventilation are important areas of research [11].

The objective of this study was to comprehensively compare, analyze, and evaluate changes in natural ventilation performance caused by greenhouse side openings. In particular, the ventilation performance was compared and analyzed according to the ventilation opening area, external wind speed, and temperature difference between the inside and outside of a greenhouse, which is mainly used in protected horticulture in South Korea.

2. Materials and Methods

2.1. Experimental Greenhouses

Figure 1 shows a schematic of the experimental greenhouse. Briefly, three single-span plastic greenhouses located at the Protected Horticulture Research Institute (35°23′ N, 128°42′ E), Gyeongsangnam-do, South Korea, were used in this study. The experimental greenhouses were installed in an east–west orientation, adjacent to each other at a distance of 3.5 m between them, and were covered with a 0.1 mm thick polyethylene (PE, Ihlshin Chemical, Ansan, Republic of Korea) film. The greenhouses were designed identically, with a floor area of 142.5 m² (7.5 × 19.0 m), eaves 1.8 m high, and ridges 3.7 m high. The experiments were conducted continuously from October 5 to 9, 2020. To analyze ventilation performance based on side-opening ventilation, the three greenhouses had different side-opening heights of 0.3, 0.7, and 1.1 m. The side openings were fitted with insect screens to prevent pests from entering and were kept open 24 h a day. The insect screens had a porosity (ratio between the open and total area of the screen) of about 0.38 (16 mesh, with a wire diameter of 0.6 mm). The greenhouses were free of roof openings, shading, thermal curtains, ventilation fans, and air-cooling and heating systems. In addition, to eliminate the effects of evapotranspiration, no crops were grown in the greenhouses.

2.2. Measurements

There were no significant fluctuations in clear weather conditions during the experiment. The data used for the comparative analysis were acquired from 9:00 to 17:00, when the amount of solar radiation was at its highest. The temperatures inside and outside the greenhouses were measured using a KCC 320 data logger (Kimo Instruments, Montpon, France) positioned at a height of 1.5 m above the ground level. The temperature sensor had a measuring range of 0 to 50 °C, with a precision of ±0.4 °C and a measurement resolution of 0.1 °C. Two and three KCC 320 data loggers were installed outdoors and indoors, respectively. The indoor temperature was measured at three points in each of the four central sections along the lengths of the greenhouses. The measured data were stored continuously at 10 min intervals and averaged.

The outdoor wind speed was measured using a 05103 wind-speed sensor (R. M. Young, Traverse, MI, USA), and solar radiation was measured using a CMP 11 solar radiation sensor (Kipp&Zonen, Delft, The Netherlands). The wind-speed and solar radiation sensors were positioned 4.0 m above the ground, and data were logged using a data logger CR1000 (Campbell Scientific, Logan, UT, USA). The wind-speed sensor had a precision of ±0.3 m s⁻¹ over a range from −50 to 50 °C, and the insolation sensor had a precision of 7 to 14 μV W⁻¹ m⁻² over a range from −10 to 40 °C. Measurements were taken every minute and averaged at 10 min intervals.

2.3. Analysis of Ventilation

The volume flow rate represents the inflow of outdoor air per unit of time [8,18]. Therefore, the natural ventilation volume can be calculated by combining the buoyancy forces due to the temperature difference between the inside and outside of the greenhouse and the wind forces due to the outdoor wind speed [19,20]. Wind forces are considered strong when their speeds exceed 2.0 m s⁻¹. Moreover, buoyancy forces are largely shaped by the temperature difference between the inside and outside of the greenhouse when the outdoor wind speed is below 1.0 m s⁻¹. When the outdoor wind speed is between 1.0 and 2.0 m s⁻¹, the wind and buoyancy forces are combined [10,18,21]. In this study, the volume flow rate was analyzed using the Bernoulli equation. The volume flow rate using the side openings can be calculated using the following Equation [22,23].

$$Q_V=\frac{A_T}{2}{C}_d{\left(2\mathrm{g}\frac{∆T}{T_O}\frac{h}{4}+C_W{u}^2\right)}^{0.5}$$

(1)

where Q_v is the volume flow rate (m³ s⁻¹), A_T is the opening area (m²), C_d is the discharge coefficient, g is the gravitational constant (m s⁻²), ΔT is the temperature difference between inside and outside (°C), T_O is the outdoor air temperature (°C), h is the opening height (m), C_w is the wind force coefficient, and u is the outside wind speed (m s⁻¹). In Equation (1), C_d is 0.253 and C_w is 0.075 [23]. Teitel et al. [23] analyzed the ventilation performance of a single-span greenhouse with two continuous side openings; the porosity of the insect screen in their study was 0.35, which is very similar to 0.38 in this study. The C_d levels were significantly different in the presence and absence of insect screens. By examining C_d without the use of insect screens, Roy et al. [22] reported an average of 0.66 and a range of 0.6–0.8. Kittas et al. [24] reported a value of 0.75.

The volume flow rate was not equal to the sum of the buoyancy forces and wind forces. The volume flow rate depends on the ratio of buoyancy and wind, and the contribution of buoyancy and wind to natural ventilation is expressed as the ratio of the total flow (buoyancy + wind) [20,21]. The proportion of wind in the total volume flow rate is given by Equation (2) and that of buoyancy is given by Equation (3) [24,25].

$$Q_W=\frac{A_T}{2}{C}_d{\left(C_W{u}^2\right)}^{0.5}$$

(2)

$$Q_T=\frac{A_T}{2}{C}_d{\left(2g\frac{∆T}{T_O}\frac{h}{4}\right)}^{0.5}$$

(3)

3. Results and Discussion

3.1. Climatic Conditions and Ventilation Performance

During the experiment, the outdoor temperature varied from 16.8 to 27.0 °C, with an average of 23.4 °C. The relative humidity ranged from 23.6 to 65.9%, with an average of 39.4%. However, the door wind speed varied from 0.2 to 4.7 m s⁻¹, with an average of 2.7 m s⁻¹. Finally, the solar radiation varied from 143.0 to 795.2 W m⁻², with an average of 568.6 W m⁻² (

Table 1. Summary of the outdoor air temperature, relative humidity, wind speed, and solar radiation in the experimental greenhouses.

Condition	Temperature (°C)	Relative Humidity (%)	Wind Speed (m s−1)	Solar Radiation (W m−2)
Minimum	16.8	23.6	0.2	143.0
Maximum	27.0	65.9	4.7	795.2
Average	23.4	39.4	2.7	568.6

).

Table 2. Summary of the side-opening heights, ventilation-area ratios, and ventilation rates in the experimental greenhouses.

Side-Opening Height (m)	Ventilation-Area Ratio	Ventilation Rate (m3 s−1 m−2)
0.3	0.08	0.0081
0.7	0.19	0.0196
1.1	0.29	0.0315

presents the variations in the ventilation-area ratio (opening area–greenhouse floor area) and ventilation rate (volume flow rate–greenhouse floor area) according to the side-opening heights of the greenhouses during the experimental period. The ventilation-area ratio is expressed as the area of ventilation openings relative to the greenhouse floor area. The larger the ventilation-area ratio, the more effective the ventilation but the weaker the greenhouse structure. In general, the minimum ventilation-area ratio required to induce sufficient natural ventilation in a greenhouse is 15–20% [24,26]. The ventilation rate is the volume flow rate according to Equation (1) divided by the floor area of the greenhouse.

Ventilation-area ratios were 0.08, 0.19, and 0.29 when the side-opening heights of the greenhouses were 0.3, 0.7, and 1.1 m, respectively, with average ventilation rates of 0.0081, 0.0196, and 0.0315 m³ s⁻¹ m⁻², respectively. As the ventilation-area ratios increased from 0.08 to 0.19 (133.3%), the ventilation rate increased by 0.0116 m³ s⁻¹ m⁻² (143.4%). The ventilation rate increased by 0.0118 m³ s⁻¹ m⁻² (60.3%) when the ventilation-area ratio increased from 0.19 to 0.29 (57.1%). The ventilation rate increased by 0.0234 m³ s⁻¹ m⁻² (290.3%) when the ventilation-area ratio increased from 0.08 to 0.29 (266.7%). Thus, it can be quantitatively confirmed that the ventilation rate increases at a rate similar to the side-opening height. That is, the ventilation-area ratio increases in the greenhouse.

3.2. Variation in Ventilation Performance with Outdoor Wind Speed

To determine the effect of wind speed on ventilation performance, the change in ventilation rate was analyzed as a function of outdoor wind speed (Figure 2). The ventilation rate tended to increase as the outside wind speed and ventilation-area ratios increased [19,27].

An analysis of the regression of the ventilation rate with outdoor wind speed revealed the following relationships: y = 0.0025x + 0.0013 (R² = 0.9953) for a ventilation-area ratio of 0.08, y = 0.0055x + 0.0047 (R² = 0.9880) for a ventilation-area ratio of 0.19, and y = 0.0082x + 0.0092 (R² = 0.9855) for a ratio of 0.29. The coefficient of determination (R²) for the regression of the ventilation rate with the outdoor wind speed is extremely high, ranging from 0.9855 to 0.9953. These results indicate that the outdoor wind speed significantly accounted for the ventilation rate’s regression. The direct examination of the effect of outdoor wind speed on the ventilation rate revealed that, as the wind speed increased by 1 m s⁻¹, the ventilation rate of the greenhouse with a ventilation-area ratio of 0.08 increased by 0.0025 m³ s⁻¹ m⁻², that of the greenhouse with a ventilation-area ratio of 0.19 increased by 0.0055 m³ s⁻¹ m⁻², and that of the greenhouse with a ventilation-area ratio of 0.29 increased by 0.0082 m³ s⁻¹ m⁻². For ventilation-area ratios of 0.08, 0.19, and 0.29, the slopes of the regression equations were 0.0025, 0.0055, and 0.0082, respectively, and the intercepts were 0.0013, 0.0047, and 0.0092, respectively. The slope and intercept of the regression equation increased as the ventilation-area ratio increased. From the slope and intercept of the regression equation, it was quantitatively confirmed that the greenhouse with a ventilation-area ratio of 0.29 exhibited the highest variable breadth in ventilation rate in response to outdoor wind speed and the highest minimum ventilation rate without accounting for the outdoor wind speed.

As shown in Figure 2, the ventilation rate is strongly positively correlated with the outside wind speed [2,23,28,29], and the scatter plot for the change in ventilation rate with the outdoor wind speed was uniformly distributed. However, for the greenhouse with a ventilation-area ratio of 0.29, with the largest side-opening area, the R² of the regression model was relatively lower, and the scatter plot was relatively wider. For the data presented in Figure 2, there is little difference in the R² of the regression equation at the overall level, but relative to the wind speed below 2 m s⁻¹, the ventilation rate is affected not only by the outdoor wind speed but also by the difference in temperature between inside and outside the greenhouse.

Figure 3 shows the ratio of wind, calculated using Equation (2), to the total ventilation performance (Equations (2) and (3)) as a function of outdoor wind speed. For the greenhouse with a ventilation-area ratio of 0.08, the proportion of wind to total ventilation performance ranged from 0.214 to 0.857, with an average of 0.710. For the greenhouse with a ventilation-area ratio of 0.19, this proportion ranged from 0.167 to 0.819, with an average of 0.657. Finally, for the greenhouse with a ventilation-area ratio of 0.29, the proportion ranged from 0.141 to 0.812, with an average of 0.638.

The regression analysis between the proportion of wind in the total ventilation performance and outdoor wind speed yielded the following relationships: y = 0.5084x^0.3580 (R² = 0.9169) for the greenhouse with a ventilation-area ratio of 0.08, y = 0.4395x^0.4268 (R² = 0.9266) for the greenhouse with a ventilation-area ratio of 0.19, and y = 0.4062x^0.4763 (R² = 0.9463) for the greenhouse with a ventilation-area ratio of 0.29. The R² of the regression equations were significantly high, at 0.9169, 0.9266, and 0.9463, respectively, and indicated a strong and positive correlation. Overall, the higher the outdoor wind speed and the lower the ventilation-area ratio, the greater the dominance of wind in the ventilation rate.

Table 3. Summary of the effect of wind forces on the total flow from the ventilation-area ratios and wind speeds in the experimental greenhouses.

Ventilation-Area Ratio	Wind Speed (m s−1)	Wind Forces/Total Flow
0.08	1.0	0.508
	2.0	0.652
0.19	1.0	0.440
	2.0	0.591
0.29	1.0	0.406
	2.0	0.565

presents an example of the contribution of wind ventilation to the total ventilation performance as a function of outdoor wind speed using the regression equations described above. For an outdoor wind speed of 1 m s⁻¹, the wind contributions were 0.508, 0.440, and 0.406 when the ventilation-window ratios were set to 0.08, 0.19, and 0.29, respectively. For an outdoor wind speed of 2 m s⁻¹, the percentages were 0.652, 0.591, and 0.565, respectively. Notably, at an outdoor wind speed of 1 m s⁻¹ and a ventilation-area ratio of 0.29, the proportion of wind to the total ventilation performance was low at 0.406. However, when the outside wind speed was 2 m s⁻¹ and the ventilation-window ratio was 0.08, the proportion of wind in the total ventilation performance was 0.652.

Figure 4 shows the proportion of wind in the total ventilation performance when the outdoor wind speed was divided into u < 1, 1 ≤ u ≤ 2, and u > 2 m s⁻¹. When the outside wind speed u was <1 m s⁻¹, the proportion of wind in the total ventilation performance was 0.439 for the greenhouse with a ventilation-area ratio of 0.08, 0.375 for the greenhouse with a ventilation-area ratio of 0.19, and 0.342 for the greenhouse with a ventilation-area ratio of 0.29. For outdoor wind speeds of 1 ≤ u ≤ 2 m s⁻¹, the percentages of wind were 0.588, 0.519, and 0.487 when the ventilation-area ratios were 0.08, 0.19, and 0.29, respectively. For wind speeds u > 2 m s⁻¹, the values increased to 0.773, 0.726, and 0.711, respectively. When the outdoor wind speeds were categorized as u < 1, 1 ≤ u ≤ 2, and u > 2 m s⁻¹, the effects of wind on ventilation performance were calculated as 0.342–0.439, 0.487–0.588, and 0.711–0.773 of the total ventilation performance, respectively.

In this study, it was determined that wind contributed to approximately 71.1% or more of the total ventilation performance for outdoor wind speeds > 2 m s⁻¹. These results are somewhat lower than those reported by Fatnassi et al. [21], who reported that wind contributed to approximately 90% of the natural ventilation for outdoor wind speeds > 2 m s⁻¹. This difference is likely because Fatnassi et al. [21] conducted their experiments in a 70 m wide greenhouse cultivating tomatoes. Nevertheless, our results are in good agreement with those of previous studies, in which wind had a dominant effect on ventilation rates when the outdoor wind speed was >2 m s⁻¹ [10,18,21].

3.3. Changes in Ventilation Performance Due to Indoor–Outdoor Air-Temperature Difference

Figure 5 shows the variation in ventilation rate according to the indoor–outdoor temperature difference, illustrating the effect of the indoor–outdoor air-temperature difference on the ventilation performance of the greenhouse. When the ventilation-area ratios were set to 0.08, 0.19, and 0.29, the indoor–outdoor air-temperature difference of the greenhouse was found to be in the range of 4.8 to 20.9, 3.3 to 16.4, and 2.4 to 12.1 °C, respectively. Moreover, the ventilation rates ranged from 0.0022 to 0.0134, 0.0069 to 0.0317, and 0.0132 to 0.0498 m³ s⁻¹ m⁻², respectively. The average indoor–outdoor temperature differences of the greenhouses with ventilation-area ratios of 0.08, 0.19, and 0.29 were 14.0, 10.1, and 7.7 °C, respectively. Moreover, the average ventilation rates were calculated as 0.0081, 0.0196, and 0.0315 m³ s⁻¹ m⁻², respectively (Table 2).

As shown in Figure 5, as the ventilation-area ratios increased to 0.08, 0.19, and 0.29, the scatter plot of the ventilation rate tended to be distributed from horizontal to vertical (Figure 5). In addition, the scatterplot distribution spread out vertically as the indoor and outdoor temperature differences decreased; however, the scatterplot distribution became more concentrated as the indoor and outdoor air-temperature differences increased. From these results, it has been concluded that the buoyancy forces (Equation (3)) resulting from the indoor–outdoor temperature difference are the main factors influencing the ventilation rate at specific levels of indoor and outdoor air-temperature differences [19].

Figure 6 shows the proportion of buoyancy in the total ventilation performance according to the indoor–outdoor temperature differences for the greenhouses. The proportion of buoyancy in the total ventilation performance tended to increase slightly as the indoor–outdoor temperature difference for the greenhouses and the ventilation-area ratios increased. However, compared to the effect of the outdoor wind speed on the wind (Figure 3), the effect of the greenhouse indoor–outdoor temperature difference on buoyancy was low [24].

Figure 7 shows the proportion of buoyancy in the total ventilation performance when the indoor–outdoor temperature difference was divided into ΔT ≤ 10 and ΔT > 10. When ΔT was ≤10, the proportion of buoyancy in the total ventilation performance was 0.267 for the greenhouse with a ventilation-area ratio of 0.08, 0.310 for the greenhouse with a ventilation-area ratio of 0.19, and 0.335 for the greenhouse with a ventilation-area ratio of 0.29. When ΔT was >10, the proportion of buoyancy was 0.294, 0.375, and 0.552 when the ventilation-area ratios were set to 0.08, 0.19, and 0.29, respectively. This quantitative analysis confirmed that the proportion of buoyancy in the total ventilation performance increased as the ventilation-area ratio and indoor–outdoor temperature difference increased. In this study, the buoyancy force was calculated to be as high as 55.2% of the total ventilation performance when the ventilation-area ratio was 0.29 and the indoor–outdoor temperature difference exceeded 10 °C.

Figure 8 shows the change in ventilation rate as a function of outdoor wind speed and ventilation-area ratio. The regression equation for the ventilation rate was y = 0.0349x + 0.0021. The R² was high (0.9676), indicating a strong positive linear relationship. These results show that the outdoor wind speed and ventilation-area ratios effectively explained the variation in the greenhouse ventilation rate [19]. Using this regression equation, to maintain a ventilation rate of 0.025 m³ s⁻¹ m⁻², the ventilation-area ratios need to be adjusted to 0.656, 0.328, 0.219, and 0.164 for outdoor wind speeds of 1.0, 2.0, 3.0, and 4.0 m s⁻¹, respectively. However, a ventilation-area ratio of 0.656 was deemed impractical for the greenhouse design, emphasizing the need to set a maximum ventilation-area ratio suitable for the greenhouse structure. Therefore, although quickly adjusting the ventilation performance of a greenhouse based on the ventilation-area ratio is challenging, effective evaluation and prediction of ventilation performance is possible. This is achievable by appropriately evaluating the ratios of greenhouse ventilation area according to changes in the outdoor wind speed using the above regression equation.

4. Conclusions

In this study, the changes in natural ventilation performance according to the ventilation opening area, outdoor wind speed, and indoor–outdoor temperature difference of the arched single-span plastic greenhouses that are mainly used in Korea were investigated. In addition, this study presents a basic technology for predicting and evaluating changes in natural ventilation performance according to the ventilation-area ratio and outdoor wind speed.

It was confirmed that the ventilation performance increased with a high ventilation-area ratio and outdoor wind speed. The ventilation rates were 0.0081, 0.0196, and 0.0315 m³ s⁻¹ m⁻² when the ventilation-area ratios were 0.08, 0.19, and 0.29, respectively. Based on the regression model of the ventilation rate on the outdoor wind speed, the slopes were 0.0025, 0.0055, and 0.0082, and the intercepts were 0.0013, 0.0047, and 0.0092 when the ventilation-area ratio was changed to 0.08, 0.19, and 0.29, respectively. The R² of the regression model was significantly high, ranging from 0.9855 to 0.9953, and the ventilation rate exhibited a strong positive correlation with the outdoor wind speed. Thus, the ventilation-area ratio and outdoor wind speed explained the variation in ventilation performance. However, the trends in ventilation performance and indoor–outdoor air-temperature differences were relatively low. The proportion of wind in the total ventilation performance was found to be high. The higher the outdoor wind speed, the lower the ventilation-area ratio. For an outdoor wind speed of 2 m s⁻¹ and a ventilation-area ratio of 0.08, the proportion of wind in the total ventilation performance was as high as 65.2%. The proportion of buoyancy increased as both the indoor–outdoor temperature difference for the greenhouses and the ventilation-area ratio increased. Buoyancy was revealed to be as high as 55.2% of the total ventilation performance when the ventilation-area ratio was 0.29 and the indoor–outdoor temperature difference exceeded 10 °C. In addition, a regression equation was established to predict changes in the greenhouse ventilation rate based on the outdoor wind speed and ventilation-area ratio: y = 0.0349x + 0.0021 (R² = 0.9676). These results suggest that the effective evaluation of ventilation performance is possible through the control of the ventilation-area ratio according to the changes in outdoor wind speed.

As of this date, most farmers practicing protected cultivation in South Korea customarily determine and use the ratio of the ventilation area without considering ventilation performance. However, it is necessary to consider the ventilation performance according to the outdoor wind speed and indoor–outdoor temperature difference, as empirically presented in this study. Additionally, it is necessary to develop a system that can maximize and effectively control the natural ventilation performance without compromising the structural safety of the greenhouse. Overall, further research is needed to verify and optimize natural ventilation performance under various crop conditions.