3.1. Radiation Components in the Greenhouses
Utilizing diffusive plastic films as greenhouse covers offers several advantages compared to conventional covering films. This is primarily due to the ability of diffusive covers to scatter a portion of the beam radiation during transmission. The presence of increased diffuse radiation within the greenhouse environment helps mitigate the harmful effects of direct sunspots on plant leaves. Moreover, it allows plants in the greenhouse to receive diffuse solar radiation from various directions, thereby promoting photosynthesis and facilitating optimal crop growth. The global and diffuse radiation fluxes outside (
Go and
Do) and inside (
Gi and
Di) of the three greenhouses (GH1/LPC, GH2/RC, and GH3/DC) were measured simultaneously during the experiment. The direct-beam components outside (
Bo) and inside the three greenhouses (
Bi) were estimated using Equations (1) and (2). For simplicity, the results from one summer day (29 June 2019) were selected to be presented. The diffuse radiation terms were corrected using the correction factors (
Section 2.1) for the outside (
FoDo) and inside (
FiDi) components. The corrected diffuse radiation fluxes outside and inside the three greenhouses are illustrated in
Figure 4a, and the direct-beam components (
Bo and
Bi) were calculated and are depicted in
Figure 4b.
Figure 4a shows that the three covers significantly enhanced the diffuse radiation inside the greenhouses compared to the outside diffuse radiation (
Fi Di >>
Fo Do). The locally produced cover (GH1/LPC) showed the highest diffusive power, followed by the diffusive cover (GH3/
DC), and then the reflective cover (GH2/RC). For comparison, the daily integrals for the diffuse radiation fluxes (
FoDo and
FiDi in
Figure 4a), for the direct beam radiation fluxes (
Bo and
Bi in
Figure 4b), and for the global radiation fluxes (
Go and
Gi) were calculated in MJ m
−2 (or in kWh m
−2). For the selected day, the outside diffuse (
FoDo) and direct beam (
Bo) radiation components were 4.8 and 16.8 MJ m
−2 (1.33 and 4.66 kWh m
−2), whereas the inside components (
FiDi and
Bi) were 7.9 and 8.3 MJ m
−2 (2.2 and 2.3 kWh m
−2) in GH1/LPC; 6.1 and 8.9 MJ m
−2 (1.7 and 2.47 kWh m
−2) in GH3/DC; and 5.8 and 8.3 MJ m
−2 (1.6 and 2.3 kWh m
−2) in GH2/RC. The ratio of diffuse to global solar radiation is usually defined as the diffuse index either outside (
FoDo/Go) or inside (
FiDi/
Gi) the greenhouse. The daily integrals for the diffuse and global radiation components outside and inside the three greenhouses were calculated. According to the diffusive power of the tested covers, σ (
Table 2), the diffuse index percentage increased from 22.2% outside the greenhouse to 49% in GH1/LPC, 41% in GH2/RC, and 42% in GH3/DC, respectively. Even though the diffusive film cover (DC) was designed mainly for the purpose of diffusion, the locally produced cover (LPC) showed a higher performance than the RC and DC in terms of their diffusion characteristics. This makes the LPC a promising option to serve effectively for crop production in greenhouses.
For further understanding the difference between the true and apparent transmittances of a cover to diffuse radiation, the well-known true transmittance (
of the cover to diffuse solar radiation was defined as the ratio between the inside and outside diffuse radiation when exposing the whole system to a diffuse radiation environment:
). However, the apparent transmittance (
) was estimated as
FiDi/F
oDo measured when exposing the system (the greenhouses and the measuring devices) to natural global solar radiation. Determining
shows the ability of the cover to diffuse direct beams during transmission. Values of
were estimated for the three greenhouses and the time course is illustrated in
Figure 5 to describe the apparent transmittances of the three greenhouses to the atmospheric diffuse (incident) radiation and that generated during transmission. By definition, the apparent transmittances (
of the three greenhouses are higher than one, because each cover converted a portion of solar beams to diffuse radiation during transmission. In
Figure 5, GH1/LPC shows a high capability to diffuse direct beam radiation, followed by GH3/DC and GH2/RC. The value of
depends mainly on the chemical composition and the type of additives that were added to the polymers during the production of each cover. The RC was characterized by the producer as an NIR-reflective cover, and it also showed diffusion property values very close to those of the DC (
Figure 5). The locally produced cover (LPC) exhibited a notable diffusive effect, which might be attributed to the inclusion of additives during the manufacturing process. These additives might serve as light diffusers, contributing to the enhanced diffusive power of this cover. In additions, these covers are specifically designed to withstand the challenging weather conditions typically found in arid regions. Based on the data in
Figure 5, the daily integrals of the apparent transmittance (
) were estimated to be 1.62, 1.27, and 1.19 for GH1/LPC, GH3/DC, and GH2/RC, respectively, compared to the true transmittance (
) values of 0.77, 0.76, and 0.65 reported in [
26] for the same plastic films.
3.2. The Microclimates in the Three Greenhouses
For comparison, the operation of wet-pad–fan systems were identical in the three tested identical greenhouses; all of them operated if the inside air temperature
Ti ≥ 25 °C. During the cultivation season, the daily averages of the outside air temperature
To and relative humidity (
RHo) and those inside the three greenhouses (
Ti and
RHi) were determined for the day- and nighttime periods, separately.
Figure 6a reveals the daily average variations of
To and
Ti inside the three greenhouses GH1/LPC, GH2/RC, and GH3/DC during the daytime periods. Similarly,
Figure 6b represents the daily average variations of these temperatures during the nighttime periods. During the daytime (
Figure 6a), the evaporative cooling combined with the effect of the cover helped maintain
Ti within the range of 35–37 °C in both GH1/LPC and GH2/RC. Notably, both covers exhibited a similar effect in this regard. However, the DC in GH3/DC could maintain
Ti at 3–5 °C lower than GH1/LPC and GH2/RC during the growing season. During the nighttime (
Figure 5b), the LPC and RC showed also a similar effect, and the DC in GH3/DC maintained
Ti at 1–3 °C lower than GH1/LPC and GH2/RC during the growing season. This means that the DC could provide the best cooling effect in GH3/DC, better than the LPC and RC. However, when referring to the
values in
Table 2, it appears that the lowest NIR heating load should have been entered GH2/RC, and the most effective cooling effect should have been observed in the case of RC in GH2/RC, rather than DC in GH3/DC. This may be attributed to the cooling performance of the wet pads in the cooling systems; even though the three cooling systems were similar, the working performance of the wet pads were different due to the accumulation of salts on each pad surface being different among the three cooling systems. In addition, several other factors can affect the wet pads’ performance such as dust accumulation, the pad location, the flow rate of the cooling water and its uniformity on the pad surface, etc.
The evaporative cooling systems in GH1/LPC, GH2/RC, and GH3/DC operated continuously during the daytime. However, during the nighttime, they were only active for a brief duration in GH1/LPC and GH2/RC. In contrast, the evaporative cooling system in GH3/DC did not operate at all during the nighttime. Therefore, the accumulative water and electric energy consumption for cooling were low in GH3/DC, being recorded at 48.7 m
3 and 826 kWh (per season) compared to 54 m
3 and 841 kWh in GH2/RC and 59.5 m
3 and 865 kWh in GH1/LPC. Even though the reduction in the inside greenhouse air temperature was relatively small, the results of the present study are in accordance with results reported in previous studies. For example, in the summer in southern Spain, a reflecting PE film covering a greenhouse showed a maximum reduction of about 4–5 °C in the
Ti values around noon [
28]. In another study in the same climate, a reflective film cover reduced the inside greenhouse air temperature (
Ti) by about 3 °C, at around noon, compared to a conventional PE cover [
29]. Also, a reflective film used to cover a greenhouse model reduced
Ti by 2 °C at noon compared to the model covered with a conventional PE film in [
30].
In arid climates, the common relative humidity is very low, averaging around 10–15% throughout both the daytime and nighttime (
Figure 7a,b). Therefore, an evaporative cooling system is essential to keep the relative humidity in greenhouses at a desired level for plant growth. However, the cooling efficiency of these systems deteriorates rapidly due to the high salinity of the cooling water used. The salinity leads to clogging of the cooling pads and subsequent reductuin in airflow rate, resulting in a decrease in overall efficiency. Due to this reason, the cooling system combined with the selective covers used in this study increased
RHi from about 10% to 30–45% during the daytime (
Figure 7a). On the other hand, during the nighttime (
Figure 7b), the covering materials maintained
RHi in the range of 50–70% during the growing season even without operating the cooling system (with the exhaust fans operating only). This assures the fact that in arid climates, growers should carefully select their greenhouse-covering materials to provide a better microclimate for crop growth requirements. Basically, the relative humidity of the inside greenhouse air (
RHi) increases with decreases in the inside air temperature (
Ti); thus, during daytime, the
RHi values in GH3/DC were slightly higher (2–3%) compared to the
RHi values in GH1/LPC, and GH2/RC maintained
RHi values 5–7% lower than those in GH1/LPC and GH3/DC during most of the growing season.
Global solar radiation transmitted into the greenhouse (
Gi) is the main source of heat energy; about 70% of G
i is transformed into sensible heat used to warm up the greenhouse air [
31]. To show the effect of the selected covers on the values of
Gi, the daily integrals of
Go and
Gi in the three greenhouses (GH1/LPC, GH2/RC, and GH3/DC) were determined (in MJ m
−2) and are depicted in
Figure 8a for the growing season. Similarly, the daily integrals of the photosynthetically active radiation (
PAR) outside and inside the greenhouses were determined (in mole m
−2) and are depicted in
Figure 8b. In
Figure 8a, G
i in GH1/LPC was the highest and in GH3/DC was the lowest depending on the cover’s transmittance to the global solar irradiance; consequently, the inside air temperature (
Ti) followed similar trends (
Figure 6a), and the inside relative humidity (
RHi) followed opposite trends (
Figure 7a). In
Figure 8b,
PARi in GH1/LPC was the highest, and that in GH3/DC was the lowest, as in the case of G
i. High levels of
PARi would enhance photosynthesis and stimulate crop growth and production. However, the
PARi levels in all the three greenhouses not only meet but also exceed the requirements for crop growth, ranging from those suitable for indoor and ornamental plants (optimum
PAR of 5 mole m
−2 day
−1) to those necessary of rice and wheat crops (optimum
PAR of 17–23 mole m
−2day
−1) [
32].
3.3. Crop Growth Parameters in the Three Greenhouses
The average values of the stem length (cm) and fresh and dry weights of the stem (gm) were estimated (as an average value per plant) in the three greenhouses (GH1/LPC, GH2/RC, and GH3/DC) and are illustrated in
Table 3. In GH3/DC, the length, fresh weight, and dry weight of the stems were significantly higher (
p ≤ 0.05) than those in GH1/LPC by about 9%, 18%, and 27%, respectively; and higher than those in GH2/RC by 5%, 15%, and 14%, respectively. In addition, measurements of the average leaf area per plant (in cm
2) and the average fresh and dry weights of the leaves (in grams) were conducted for the three greenhouses. The corresponding results are presented in
Table 4. In GH3/DC, the area, fresh weight, and dry weight of the leaves are significantly higher (
p ≤ 0.05) than those in GH1/LPC by about 18%, 12%, and 18.5%, respectively; and higher than those in GH2/RC by about 12%, 5%, and 11%, respectively. The data in
Table 3 and
Table 4 indicate that the diffusive cover (DC) significantly improved (
p ≤ 0.05) the vegetative growth of the cucumber crops, which was reflected in their biomass, followed by the reflective cover (RC) and the locally produced cover (LPC), in that order.
3.4. Crop Yield in the Three Greenhouses
Among the three greenhouses tested, the vegetative growth, total dry biomass, and crop yield followed a similar manner as affected by the microclimatic conditions that were generated based on the type of cover and its radiative properties. The number of fruits (per m
2), the fruit length, and the fresh and dry weights of the fruits increased for plants grown in GH3/DC, with significant differences (
p ≤ 0.05) from both those grown in GH1/LPC and those grown in GH2/RC (
Table 5). The fruit length, diameter, and fresh and dry weights were 9%, 7.6%, 13%, and 11.8% higher (
p ≤ 0.05) than those in GH1/LPC, and 6%, 11%, 6%, and 8% higher than those in GH2/RC. Additionally, the number of fruits in GH3/DC was 108.5, which was 8% higher (
p ≤ 0.05) than that in GH1/LPC and 14% higher than that in GH2/RC. The crop yield values were 12.3, 10.2, and 10.1 kg m
−2 for GH3/DC, GH2/RC, and GH1/LPC, respectively.
The presented results indicated that the growth and yield traits increased in GH3/DC, followed by GH2/RC and then by GH1/LPC. This is because during the growing season, the daily averages of the inside air temperature (
Ti) and relative humidity (
RHi) were optimal for cucumber growth and development in GH3/DC as compared to those in GH1/LPC and GH2/RC. Even though the PAR transmittance and the diffusive power of the locally produced cover (LPC) were higher than those of the other two covers (DC and RC), the crop growth behavior and productivity depended mainly on
Ti and
RHi. The relatively low
Ti and high
RHi in GH3/DC (
Figure 6a,b and
Figure 7a,b) led to a 22% increase in productivity as compared with GH1/LPC and a 20% increase in comparison with GH2/RC. It is important to note that the
PARi levels in all of the three greenhouses were sufficient to support the growth and development of cucumber crops, as shown in
Figure 8b. Finally, under the same climatic conditions, the quality, quantity, and productivity of the plants in each greenhouse depended on a combination of different factors, such as the inside air temperature and relative humidity (
Ti,
RHi), photosynthetically active radiation (
PARi), diffuse radiation (
Di), ventilation rate, pathogenic activity, etc. These factors together depended mainly on the radiative and thermophysical properties of the covering material and on the performance of the evaporative cooling system. In addition, the price and the service life of the tested covers are different (
Table 1); therefore, it is quite difficult to evaluate the financial impact of these covers at the moment.