Spatial distribution of air temperature and relative humidity in the greenhouse as affected by external shading in arid climates

Journal of Integrative Agriculture 2019, 18(12): 2869–2882 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Spatial distribution of air temperature and relative humidity in the greenhouse as affected by external shading in arid climates Hesham A. Ahmed1, 2, 3, 4, TONG Yu-xin1, 2, YANG Qi-chang1, 2, Abdulellah A. Al-Faraj3, Ahmed M. AbdelGhany3 1

Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R.China 2 Key Laboratory of Energy Conservation and Waste Management of Agricultural Structures, Beijing 100081, P.R.China 3 Department of Agricultural Engineering, College of Food and Agriculture Sciences, King Saud University, Riyadh 11451, Saudi Arabia 4 Department of Agricultural Engineering, Faculty of Agriculture, Sana’a University, Sana’a 2124, Yemen

Abstract The effect of external roof shading on the spatial distribution of air temperature and relative humidity in a greenhouse (Tin and RHin) was evaluated under the arid climatic conditions of Riyadh City, Saudi Arabia. Two identical, evaporatively-cooled, single-span greenhouses were used in the experiment. One greenhouse was externally shaded (Gs) using a movable black plastic net (30% transmissivity), and the other greenhouse was kept without shading (Gc). Strawberry plants were cultivated in both greenhouses. The results showed that the spatial distribution of the Tin and RHin was significantly affected by the outside solar radiation and evaporative cooling operation. The regression analysis showed that when the outside solar radiation intensity increased from 200 to 800 W m–2, the Tin increased by 4.5°C in the Gc and 2°C in the Gs, while the RHin decreased by 15% in the Gc and 5% in the Gs, respectively. Compared with those in the Gc, more uniformity in the spatial distribution of the Tin and RHin was observed in the Gs. The difference between the maximum and minimum Tin of 6.4°C and the RHin of 10% was lower in the Gs than those in the Gc during the early morning. Around 2°C difference in the Tin was shown between the area closed to the exhausted fans and the area closed to the cooling pad with the external shading. In an evaporatively-cooled greenhouse in arid regions, the variation of the Tin and RHin in the vertical direction and along the sidewalls was much higher than that in the horizontal direction. The average variation of the Tin and RHin in the vertical direction was 5.2°C and 10% in the Gc and 5.5°C and 13% in the Gs, respectively. The external shading improved the spatial distribution of the Tin and RHin and improved the cooling efficiency of the evaporative cooling system by 12%, since the transmitted solar radiation and accumulated thermal energy in the greenhouse were significantly reduced. Keywords: greenhouse, temperature, humidity, distribution, uniformity, evaporative cooling, shading, arid climate

Received 15 August, 2018 Accepted 3 December, 2018 Hesham A. Ahmed, Mobile: +86-18516866350, E-mail: [email protected]; Correspondence TONG Yu-xin, Mobile: +86-18600218498, E-mail: [email protected] © 2019 CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). doi: 10.1016/S2095-3119(19)62598-0

1. Introduction The distribution of air temperature and relative humidity in a greenhouse (T in and RH in) are two of the main factors influencing the uniformity of plant growth and crop

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productivity (Teitel et al. 2001, 2010; Bojaca et al. 2009a, b). For this reason, studying the spatial distribution of the Tin and RHin is highly important to provide useful information for monitoring and controlling the greenhouse environment (Ryu et al. 2014). It also leads to an improved design in the ventilation and heating or cooling systems (Van Pee et al. 1998; Ahemd et al. 2016). The distribution of the Tin and RHin depends on various factors, including changes in external climate, solar radiation intensity, cooling systems, ventilation methods, covering and shading materials, the presence of the plant canopy, and the dimensions of the greenhouse (Bailey 1991; Bucklin et al. 1993; Zhao et al. 2001; Bartzanas and Kittas 2005; Bojaca et al. 2009a, b; Ryu et al. 2014). The solar radiation transmitted into the greenhouse is the primary source of the sensible heat, which leads to an increase in the Tin and its variation within the greenhouse (Sethi and Sharma 2007; Ahmed et al. 2016). As a result, the highest variations in the Tin and RHin are usually observed during noon time (Sapounas et al. 2008a; Ryu et al. 2014). A cooling system, such as evaporative cooling which consists of wet pad and fans, can maintain the Tin at 8–10°C lower than air temperature outside the greenhouse (Tout). However, one of the main operational disadvantages of the evaporative cooling system is creating a large variation in the Tin and RHin as a result of air movement along the pad-fans direction (Arbel et al. 1999; Al-Helal and Al-Musalam 2003; Kittas et al. 2003; Bartzanas and Kittas 2005; Sethi and Sharma 2007; Kumar et al. 2009; Ganguly and Ghosh 2011). The ventilation system is another main reason that affects the distribution of the microclimatic parameters in a greenhouse. Increasing the ventilation rate leads to an increase in the vertical variation of the Tin and RHin and an increase in plant stress (Seginer 1994; Li and Willits 2008; Chen et al. 2011). Sapounas et al. (2008b) found that the vertical variation of the Tin in a naturally ventilated greenhouse can reach 10°C. Arbel et al. (2003) reported that the horizontal direction of ventilation in a fan-ventilated greenhouse makes the horizontal distribution of the Tin and RHin more uniform than that on the vertical direction. Shading also plays a key role in improving the distribution of the Tin and RHin. Shading reduces the transmitted solar radiation and can maintain the Tin at a suitable level for plant growth. There are many ways and materials can be used to shade the greenhouse such as whitewash, retractable liquid foam, flexible solar panel, and plastic shading nets. In the last decades, using plastic shading nets as shading materials was expanded. Plastic shading nets can be mounted outside or inside the greenhouse with a fixed or movable way and have the ability to scatter the transmitted solar radiation inside the greenhouse. The scattering of solar radiation improves the uniformity of light and consequently,

the uniformity of the Tin and RHin will be improved (AbdelGhany and Al-Helal 2011; Ahemd et al. 2016). Salokhe et al. (2005a, b) stated that internal shading along the greenhouse sidewalls reduces the ventilation rate due to creating resistance to the air exchange between the inside and the outside environment, and thus negatively affect the microclimate uniformity. Using shading inside the greenhouse also has a negative effect on the cooling owing to thermal radiation emitted inside the greenhouse (AbdelGhany et al. 2015). Therefore, combining external shading method with a cooling strategy, such as evaporative cooling or ventilation, is a preferable way to improve the uniformity of the Tin and RHin (Willits and Peet 2000; Kittas et al. 2001; Bartzanas and Kittas 2005). Thus, the main objective of this study is to evaluate the spatial distribution of the Tin and RHin as affected by external shading together with an evaporative cooling system in the arid climate. The evaluation includes a comparison between two greenhouses with and without external shading and provides a description for the Tin and RHin distribution along the greenhouse length, width, and height. However, the effects of the boundary conditions (i.e., floor surface and cover temperatures) on the spatial distribution of Tin and RHin is beyond the scope of the present study.

2. Materials and methods 2.1. Greenhouses and experimental setup The experiment was conducted in two identical singlespans, curved-roof greenhouses, each with a floor area of 160 m2 (8 m×20 m), covered with a double-layer, hollowchanneled, polycarbonate sheet of 8.15-mm thickness, a gutter height of 4 m and a gable height of 2.5 m. The greenhouses were oriented in the North (N)–South (S) direction on the Agricultural Research and Experiment Station, Agricultural Engineering Department, King Saud University (Riyadh, Saudi Arabia, 46°47´E, and 24°39´N). The radiative properties of the new polycarbonate cover are: the transmittance to global solar radiation (200–2 500 nm) is 70%, to the photosynthetically active radiation (PAR: 400–700 nm) is 71%, and to the IR-thermal radiation (2 500–12 500 nm) is 1% (Abdel-Ghany et al. 2015). The greenhouses were cooled with two identical evaporative cooling systems (wet-pad and fans). Each includes a cellulose cooling pad of 14 m2 surface area, 0.10-m thick and was mounted on the north side-wall. On the south side-wall, two identical exhaust fans were mounted at 0.75 m above the greenhouse floor. The distance between the fans is 6 m. Each fan with a diameter of 1.2 m, an air flow rate of 350 m3 min–1 and power consumption of 1.1 kW. The cooling system is controlled by a control unit, where

Hesham A. Ahmed et al. Journal of Integrative Agriculture 2019, 18(12): 2869–2882

the cooling stage can be set up to run either automatically or manually. During the experiment, the cooling system was controlled to operate if the Tin reaches to a set point temperature of 28°C. Strawberry is a common crop in Saudi Arabia during the period of the experiment (from Oct. 16th to 26th, 2017). Therefore, the strawberry crop was cultivated in both greenhouses with a density of 6 plants m–2. The cultivation was in pots and drip irrigation was applied by using a pre-installed irrigation system. To perform the experiment, one greenhouse was kept without shading as control (Gc), and the other was externally shaded (Gs) using a movable black plastic net during the daytime period. The radiative properties of the new plastic net are: the transmittance to global solar radiation (200– 2 500 nm) is 30%, to the photosynthetically active radiation (PAR: 400–700 nm) is 27.5%, and to the IR-thermal radiation (2 500–12 500 nm) is 23% (Abdel-Ghany et al. 2015). The external shading was deployed at a height of 1-m above the greenhouse roof throughout the period of the experiment. Photograph for the shaded and unshaded greenhouses is illustrated in Fig. 1-A.

2.2. Measurements and calculations Air temperature and relative humidity The Tin and RHin were measured at two horizontal levels, namely, at 1-m (H1) and 3-m (H2) above the greenhouse floor. Each horizontal level contained nine measuring locations, which were uniformly distributed as shown in Figs.1-B and 2. The Tin and RHin (at 18 locations) and the Tout and RHout (at 4-m above the floor level and a distance of 6-m from the greenhouses) were measured using combined temperature-humidity-data-logger sensors (OM-EL-USB2-LCD, Omega Inc., USA). The sensors have a maximum error of ±0.5°C and ±3% for the air temperature and relative humidity, respectively; a working temperature range of –35 to +80°C and a relative humidity range of 0 to100%. The sensors were protected against the radiation effect and calibrated before use by means of the supplier. All values were measured at 1-min intervals, averaged and recorded at each 5 min. Solar radiation The inside and outside global solar radiation (SRin and SRout) were measured using CMP3 pyranometers (Kipp&Zonen B.V. Inc., USA), having a time response of 18 µs, a maximum error of ±1%, a sensitivity of 5–20 µV W–1 m–2, a working temperature range of –40 to +80°C and a wavelength range of 300 to 2 800 nm. The SRin sensor was placed at the center of the greenhouse on the top of a wooden box of 1.5 m in height. The SRout sensor was mounted on a metallic mast of 4 m height above the ground level. Values of SRin and SRout were measured at 30-s intervals, averaged at each 5 min and recorded in

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a data logger (CR3000 Micro logger, Campbell Scientific, Inc., USA). The required parameters were measured in 10 consecutive sunny days, no significant differences were observed in the values of each parameter among these days. Therefore, for each parameter, the measured data were averaged to be represented in one day and depicted in figures. Estimation of cooling efficiency The cooling efficiency (η, %) of the evaporative cooling system was determined as the ratio between the drop-in air temperature after passing through the cooling pad and the maximum drop under air saturation conditions. Todb–Tidb (1) η (%)= ×100 Todb–Towb

where Todb is the outside dry-bulb temperature of air entering the pad (°C), Tidb is the dry-bulb temperature of air leaving the pad (°C), and Towb is the outside wet-bulb temperature of air entering the pad. The Todb and Tidb were measured directly. The Tidb representing the average of three points was measured in the front of the cooling pad. The Towb was calculated using Jaramillo et al. (2014) empirical models (Table 1). Statistical analysis The spatial distribution of the Tin and RHin was described and analyzed based on three directions: two on the horizontal level and one in the vertical level. Fig. 2 is a 3-D diagram to illustrate the locations of the measuring points and the ID number of each point and all points are summarized in Table 2. Descriptions of these directions are as follows: (i) Horizontal distribution along the greenhouse length (cooling pad-fans direction). Each greenhouse in the N–S direction was divided into three sections from the cooling pad to the fans. Each section was 6 m×2 m in size. The first section was located near to the cooling pad (CP), the second section was located in the middle of the greenhouse (ML), and the third section was located near to the exhaust fans (F). Illustration of the section’s IDs and the related IDs of the measuring point are summarized in Table 2. (ii) Horizontal distribution along the greenhouse width (the east (E)–west (W) direction). Each greenhouse was divided, in the E–W direction, into three sections. Each section was 16 m×2 m in size. The first section was located on the east side (E), the second section was located in the middle of the greenhouse (MW), and the third section was located on the west side (W, Table 2). (iii) Vertical distribution along the height of the greenhouse. Each greenhouse was divided into two vertical sections. Each section was 16 m×6 m in size. The first section was located at 1-m height (H1) and the second section was located at 3-m height (H2) above the greenhouse floor (Table 2). The average values of the Tin and RHin for each section in the horizontal and vertical directions

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A

N

Cooling pad

W

1m

E

3m

4m 2m

8m

16 m

S

20 m

B

Fans ×

× 6m

6m

8m

8m

Fig. 1 Greenhouses used in the experiment and experimental setup. A, photograph for the experimental greenhouses. B, a schematic diagram showing the horizontal (right side) and vertical (left side) locations of the measuring points of air temperature and relative humidity.

were estimated. The obtained results were analyzed and compared using a general linear model (CRD) at a 5% probability using SAS Software (Ver. 8.1; SAS Institute, Cary, NC, USA).

3. Results 3.1. Arid climate and greenhouse environmental parameters Solar radiation Almost similar SRout was observed during the days of the experiment because the clear sky condition is the prevailing climate in arid regions. The maximum SRout reached 813 W m–2 at 12:00 (Fig. 3-A), and the daily average of the SRout was 417 W m–2 (Table 3). The average SRin in the Gc of 209 W m–2 was reduced by 50% compared to

SRout. The average SRin in the Gs of 106 W m–2 was reduced by 75% compared to SRout due to the combining effect of greenhouse cover and the external shading. Air temperature The average air temperature in the unshaded greenhouse (Tin-c) and in the shaded greenhouse (Tin-s) from 10:00 to 15:00 was 32 and 29°C, respectively, which were 4 and 7°C lower than To (Fig. 3-B). A regression analysis showed that when the SRout increased from 200 to 800 W m–2, the Tin-c increased from 28.0 to 32.5°C (R2=0.94) and the Tin-s increased from 28.0 to 30.0°C (R2=0.53), respectively. Both Tin-c and Tin-s were slightly affected by the SRout in the early morning and late afternoon periods, while they were drastically affected during the noon time. Based on the variance analysis of the Tin-c and Tin-s, the combined effect of external shading and the pad-fans cooling system had a significant effect (P=0.05) on reducing the Tin in the

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16 17 18

7 14

8 15

9

6m

6

5

13

4

10 2m

11

1

12 2 3

16 m

Fig. 2 The statistical design of the air temperature and relative humidity measuring points.

Table 1 Empirical equations to estimate the wet-bulb temperature Relative humidity range (%) 11–15 16–20 21–25 26–30 31–35 36–40 1)

Wet-bulb temperature equation (°C)1) Twb=0.5506Tdb–3.5845 Twb=0.593Tdb–3.5889 Twb=0.6456Tdb–3.8737 Twb=0.6845Tdb–3.8201 Twb=0.7277Tdb–3.9253 Twb=0.7555Tdb–3.6281

R2 0.8711 0.9144 0.9643 0.9828 0.9868 0.9915

Twb, wet-bulb temperature; Tdb, dry-bulb temperature.

arid climate greenhouses (Table 3). Relative humidity The relative humidity in the unshaded greenhouse (RHin-c) and in the shaded greenhouse (RHin-s) from 6:00 to 8:30 was uneven. The RHin-c decreased from 84 to 55% whereas the RHin-s decreased from 73 to 52% (Fig. 3-C). The average RHin-c and RHin-s from 8:30 to 18:00 were around 50%, and drastically affected by the intensity of the SRout. The RHin-c decreased from 75 to 55% (R2=0.94) and the RHin-s decreased from 60 to 55% (R2=0.84) when the SRout increased from 200 to 800 W m–2. Based on the statistical analysis of the results in Table 3, the RHin-c of 54.4% was significantly higher than the RHin-s of 52% at

P=0.05. Cooling efficiency The Tin after the cooling pad was compared with the To as an important criterion to calculate the cooling efficiency of the pad-fans cooling system. Throughout the period of the experiment, the cooling efficiency of the pad-fans cooling system in the Gs was always higher than that in the Gc (Fig. 3-D). The average value of the cooling efficiency was 45% in the Gc and 57% in the Gs, respectively. The cooling efficiency of the pad-fans cooling system in the Gs increased by 14% compared to that in the Gc when the SRout reached 800 W m–2. The above results indicated that the cooling efficiency in arid climate greenhouses significantly improved by combing external shading with the pad-fans cooling system.

3.2. The horizontal distribution of the Tin and RHin along the pad-fans direction At 1-m height above the ground The T in-c and T in-s increased with the increase of distance from the cooling pad (Fig. 4-A and B). The Tin-c at the exhausted fans reached the highest value of 32.8°C at noon time. It was 4.3°C higher than that at the area near to the cooling pad (Table 4).

Table 2 The measuring location and section IDs of air temperature and relative humidity in both greenhouses Distribution of the measuring points Horizontal distribution along the greenhouse length

Horizontal distribution along the greenhouse width

Vertical distribution along the height of the greenhouse 1) 2)

At 1-m height Measuring location ID Section ID1) 1, 4, 7 CP 2, 5, 8 ML 3, 6, 9 F 1, 2, 3 E 4, 5, 6 MW 7, 8, 9 W 1–9 H1

At 3-m height Measuring location ID Section ID2) 10, 13, 16 CP 11, 14, 17 ML 12, 15, 18 F 10, 11, 12 E 13, 14, 15 MW 16, 17, 18 W 10–18 H2

CP, cooling pad; ML and MW, the middle of the greenhouse; F, exhausted fans; E, east side; W, west side; H1, 1-m height. H2, 3-m height.

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Outside

Gc B 40

1 000 800

Air temperature (°C)

Solar radiation (W m–2)

A

600 400 200

Gs

35 30 25

0 D 80

100

Cooling efficiency (%)

Relative humidity (%)

C

80 60 40 20 0 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

60 40 20 Gc Gs 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

Fig. 3 Diurnal variation of the average values of solar radiation flux (A), air temperature (B), relative humidity (C) measured outside and inside the shaded and unshaded greenhouses and the cooling efficiency (D) during the period of the experiment (16th to 26th, Oct. 2017). Gc, unshaded greenhouse; Gs, shaded greenhouse. Table 3 Daily variation of the climatic parameters measured outside and inside the greenhouses during the period of the experiment (16th to 26th, Oct. 2017) Variable1) SR (W m–2) T (°C) RH (%)

Outside climate Mean±SD Min. 417.0±280.0 0.0 34.0±3.9 23.9 22.4±6.4 15.4

Max. 813.0 39.9 37.8

Unshaded greenhouse Mean±SD Min. Max. 209.0±120.0 a 0.0 342.0 30.0±2.4 a 23.9 32.8 54.4±11.4 a 44.8 84.5

Shaded greenhouse Mean±SD Min. Max. 106.0±48.0 b 0.0 231.0 28.5±1.6 b 23.8 30.2 52.0±6.8 b 46.6 73.3

1)

SR, solar radiation; T, air temperature; RH, relative humidity. Different letters indicate significant differences between the solar radiation, air temperature and relative humidity averages in the unshaded and shaded greenhouses at P=0.05.

The highest Tin-s was 27.3 and 28.6°C near the cooling

distribution of the RHin-s along the pad-fans direction.

pad and the exhausted fans, respectively (Table 4). Based

At 3-m height above the ground The uniformity of the Tin

on the analysis of variance in Table 4, the distribution of the

and RHin distribution along the pad-fans direction decreased

Tin-c and Tin-c varied significantly (P=0.05) along the pad-fans

with the increase of height above the greenhouse floor. The

direction.

Tin got higher (Fig. 4-C and D) and RHin got lower than those

The RH in in both greenhouses decreased with the

at 1-m height (Fig. 5-C and D). The highest Tin-c of 35.8°C

increase of distance from the cooling pad. The highest

and the lowest RHin-c of 38.3% were recorded in the middle

horizontal variation along the pad-fans direction was

of the greenhouse at noon time (Table 5).

observed at noon time (Fig. 5-A and B). The RHin was 5

The highest Tin-s of 33.4°C and the lowest RHin-s of 39.2%

and 2% at the area close to the exhausted fans lower than

were recorded close to the exhausted fans (Table 5). The

the area close to the cooling pad in the Gc and the Gs,

horizontal distributions of the Tin-s and RHin-s were more

respectively. The results in Table 4 showed that the RHin-c

uniform than those in the Gc. Based on the analysis of

distribution varied significantly along the pad-fans direction.

variance, significant differences (P=0.05) were observed on

Conversely, no significant differences were observed on the

the distribution of the Tin-c and the Tin-s along the pad-fans

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Table 4 The horizontal distribution of air temperature and relative humidity along the pad-fans direction at 1-m height during the period of the experiment (16th to 26th, Oct. 2017) Measuring location1)

Variable Air temperature (°C)

Relative humidity (%)

CP ML F CP ML F

Unshaded greenhouse Mean±SD Min. 27.0±1.15 b 23.7 29.2±1.97 a 24.1 29.4±1.86 a 24.2 61.5±9.0 a 53.5 57.2±10.3 b 47.2 56.9±9.7 b 48.1

Max. 28.5 31.6 32.8 85.3 83.9 84.7

Shaded greenhouse Mean±SD Min. 25.9±0.71 b 23.7 27.3±1.02 a 23.9 27.6±1.06 a 24.2 58.3±5.0 a 51.0 57.4±5.4 a 50.6 58.0±5.2 a 51.9

Max. 27.3 28.6 28.6 73.4 73.4 74.6

1)

CP, cooling pad; ML, in the middle of the greenhouse; F, exhausted fans. Different letters indicate significant differences between the air temperature and relative humidity averages at P=0.05.

CP

Tin (°C)

C

ML

F

B 32

30

30 Tin (°C)

32

28 26

28 26

24

24

22

22 D

36

36

34

34

32

32

30

30

Tin (°C)

Tin (°C)

A

28

28

26

26

24

24

22 06:00

08:00

10:00

12:00 Time

14:00

16:00

18:00

22 06:00

08:00

10:00

12:00 Time

14:00

16:00

18:00

Fig. 4 Diurnal variation of the average values of air temperature (Tin) measured along the pad-fans direction, in the shaded (Gs) and unshaded (Gc) greenhouses, during the period of the experiment (16th to 26th, Oct. 2017). A, at 1-m height in the Gc. B, at 1-m height in the Gs. C, at 3-m height in the Gc. D, at 3-m height in the Gs. CP, Tin at the cooling pad; ML, Tin in the middle of the greenhouse; F, Tin at the exhausted fans.

direction (Table 5). However, no significant differences were observed in the distribution of the RHin-c and the RHin-s along

3.3. The horizontal distribution of the Tin and RHin along the E–W direction

the pad-fans direction (Table 5). Fluctuations were observed in the plotted air temperature and relative humidity, in the period from 8:00 to 14:00 near to the exhaust fans zone. This mainly because the suction effect of the fans makes

At 1-m height above the ground Different Tin distribution pattern was found along the E–W direction in both greenhouses (Fig. 6-A and B). The Tin distribution along the

the air movement in the fans zone is turbulence, and the

E–W direction was less uniform than that along the pad-fans

measured parameters were recorded at every 5 min. This

direction. The highest Tin-c and Tin-s were found along the

fluctuation was higher at 1-m height (in the level of the fans)

sidewalls whereas the lowest were found in the middle of

than at 3-m height.

the greenhouse (Table 6).

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CP

RHin (%)

C

ML B

90

F 90

80

70

70

RHin (%)

80

60

60

50

50

40

40 D

90

90

80

80

70

70

RHin (%)

RHin (%)

A

60

60

50

50

40

40

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

Fig. 5 Diurnal variation of the average values of relative humidity (RHin) measured along the pad-fans direction, in the shaded (Gs) and unshaded (Gc) greenhouses, during the period of the experiment (16th to 26th, Oct. 2017). A, at 1-m height in the Gc. B, at 1-m height in the Gs. C, at 3-m height in the Gc. D, at 3-m height in the Gs. CP, RHin at the cooling pad; ML, RHin in the middle of the greenhouse; F, RHin at the exhausted fans. Table 5 The horizontal distribution of air temperature and relative humidity along the pad-fans direction at 3-m height during the period of the experiment (16th to 26th, Oct. 2017) Variable Air temperature (°C)

Relative humidity (%)

Measuring location1) CP ML F CP ML F

Unshaded greenhouse Mean±SD Min. 30.8±3.12 b 23.5 31.8±3.39 a 23.9 31.8±3.19 a 23.8 50.6±13.5 a 38.8 50.1±13.5 a 38.3 50.0±12.8 a 39.0

Max. 34.4 35.8 35.5 85.0 84.6 84.2

Mean±SD 29.8±2.3 b 30.6±2.3 a 31.0±2.5 a 45.6±8.6 a 46.4±8.6 a 45.1±8.8 a

Shaded greenhouse Min. 23.2 23.9 23.9 38.8 39.8 39.2

Max. 32.0 32.8 33.4 71.8 72.9 72.4

1)

CP, cooling pad; ML, in the middle of the greenhouse; F, exhausted fans. Different letters indicate significant differences between the air temperature and relative humidity averages at P=0.05.

The difference between the highest and the lowest Tin-c was 8.4 and 7.4°C on the east and west sides, respectively. The Tin-s was reduced by the external shading to 5.3°C on the east side and 4.9°C on the west side, respectively. The analysis of variance showed a significant change (P=0.05) in the distribution of the Tin in both greenhouses, especially on the eastern side corresponding to the sunrise and the western side corresponding to the sunset. Comparison of the average values in Table 6 showed that the lowest Tin-c of 28.2°C and the lowest Tin-s of 26.5°C were recorded in the middle of the greenhouse. The RHin distribution pattern was similar to the distribution

pattern along the pad-fans direction (Fig. 7-A and B). That means no effect for the greenhouse orientation on the distribution of the RHin. The obtained results showed that the highest RHin-c of 59.5% and the highest RHin-s of 60.4% were recorded in the middle of the greenhouse (Table 6). Based on the analysis of variance in Table 6, uniform distribution was found for the RHin-c and RHin-s along the E–W direction in both greenhouses. At 3-m height above the ground The effect of SRout and the greenhouse orientation on the Tin distribution became much higher with the increase of height above the greenhouse floor (Fig. 6-C and D). The differences between Tin-c values

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E 32

32

30

30

28 26

W

28 26

24

24

22

22

C

D 36

36

34

34

32

32

30

Tin (°C)

Tin (°C)

MW B

Tin (°C)

Tin (°C)

A

28

30 28

26

26

24

24

22 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

22 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

Fig. 6 Diurnal variation of the average values of air temperature (Tin) measured along the east–west direction, in the shaded (Gs) and unshaded (Gc) greenhouses, during the period of the experiment (16th to 26th, Oct. 2017). A, at 1-m height in the Gc. B, at 1-m height in the Gs. C, at 3-m height in the Gc. D, at 3-m height in the Gs. E, Tin on the east side; MW, Tin in the middle of the greenhouse; W, Tin on the west side. Table 6 The horizontal distribution of air temperature and relative humidity along the east-west direction at 1-m height during the period of the experiment (16th to 26th, Oct. 2017) Variable Air temperature (°C)

Relative humidity (%)

Measuring location1) CP MW F CP MW F

Unshaded greenhouse Mean±SD Min. 28.8±1.8 a 24.0 28.2± 1.8 b 23.9 28.7±1.8 a 24.0 57.9±9.9 a 49.1 59.5±9.4 a 51.0 58.2±9.7 a 48.2

Max. 32.4 29.9 31.4 85.4 85.0 83.6

Shaded greenhouse Mean±SD Min. 27.3±1.1 a 24.0 26.5±0.8 b 23.9 27.1±1.2 a 24.0 57.0±5.2 a 48.8 60.4±4.9 a 53.6 57.3±5.5 a 50.6

Max. 29.3 28.2 28.9 72.5 75.1 74.1

1)

CP, cooling pad; MW, in the middle of the greenhouse; F, exhausted fans. Different letters indicate significant differences between the air temperature and relative humidity averages at P=0.05.

were ±3.6°C and the highest Tin-c reached 8.63°C on the west side at noon time. The external shading reduced the difference between the Tin values to ±2.8°C and the highest Tin was also recorded on the west side of 34.9°C (Table 7). The analysis of variance showed a limited effect for the external shading on reducing the Tin at 3-m height. The Tin-s of 1.2°C was lower than the Tin-c in the middle of the greenhouse (Table 7). Increasing the height above the greenhouse floor resulted in an irregular distribution of the RHin. The obtained results showed that the lowest RHin values in both greenhouses were found at noon time (Fig. 7-C and D). However, the distribution of the RHin-s was

more uniform than the distribution of the RHin-c as shown in Table 7. Generally, the RHin-s of 5% was lower than the RHin-c. Statistically, the results of the analysis of variance showed no significant differences (P=0.05) in the RHin-c and the RHin-s distribution along the E–W direction (Table 7).

3.4. The vertical distribution of the Tin and RHin The intensity of SRout has a major effect on the vertical distribution of the Tin and RHin in arid region’s greenhouses. Throughout the experiment period, the Tin increased and the RHin decreased with the increase of height above the

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E

MW

W

90

B 90

80

80

70

70

RHin (%)

RHin (%)

A

60

60

40

40

C 90

D 90

80

80

70

70

RHin (%)

50

RHin (%)

50

60 50

60 50

40

40

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

Fig. 7 Diurnal variation of the average values of the relative humidity (RHin) measured along the east-west direction, in the shaded (Gs) and unshaded (Gc) greenhouses, during the period of the experiment (16th to 26th, Oct. 2017). A, at 1-m height in the Gc. B, at 1-m height in the Gs. C, at 3-m height in the Gc. D, at 3-m height in the Gs. E, RHin on the east side; MW, RHin in the middle of the greenhouse; W, RHin on the west side. Table 7 The horizontal distribution of air temperature and relative humidity along the east-west direction at 3-m height during the period of the experiment (16th to 26th, Oct. 2017) Variable Air temperature (°C)

Relative humidity (%)

Measuring location1) CP MW F CP MW F

Unshaded greenhouse Mean±SD Min. 32.2±3.6 a 23.7 31.8±3.3 a 23.8 32.1±3.8 a 23.6 48.0±13.9 a 36.9 50.3±13.2 a 39.4 48.4±14.1 a 36.6

Max. 35.9 35.3 36.8 84.1 85.0 84.1

Shaded greenhouse Mean±SD Min. 31.6±2.8 a 23.7 30.6±2.5 b 23.9 31.5±3.0 a 23.8 43.0±9.0 a 36.7 45.5±9.0 a 38.7 44.6±9.6 a 37.1

Max. 34.8 33.1 34.9 71.2 72.6 73.3

1)

CP, cooling pad; MW, in the middle of the greenhouse; F, exhausted fans Different letters indicate significant differences between the air temperature and relative humidity averages at P=0.05.

greenhouse floor. The highest vertical variation of the Tin and RHin was observed around noon time (Fig. 8). The

The increasing of SRout from 200 to 800 W m–2 reduced

the RHin-c and RHin-s at 3-m height to 40% with R2=0.84 and

highest Tin-c was 30.7°C at 1-m height and 35.9°C at 3-m

0.48%, respectively. The distribution of the Tin and RHin at

recorded at 1- and 3-m height, respectively (Table 8). The

greenhouses. Based on the analysis of variance in Table 8,

regression analysis showed that increasing the SRout from

significant differences were observed between the values

height (Table 8). The highest Tin-s of 28.2 and 33.7°C were

200 to 800 W m increased the Tin-c at 3-m height from 29 to –2

35.9°C (R2=0.97) and from 29 to 33.2°C (R2=0.84) in the Gs. The RHin-c of 55 and 40% were recorded at 1- and 3-m

heights, respectively (Fig. 8-C). In the Gs, the RHin was

57% at 1-m and 40% at 3-m height, respectively (Fig. 8-D).

3-m height was less uniform than that at 1-m height in both

of the Tin and RHin at 1-m height and those at 3-m height.

4. Discussion The high SRout (1 000 W m–2) and Tout (up to 47°C) with low

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RHout (below 20%) are the most important characteristics

because of the evaporative cooling effect in decreasing Tin

and RHin levels and their variations within the greenhouse

Musalam 2003; Al-Helal 2007). The comparison in Fig. 3-B

of the arid climate (Al-Helal and Al-Musalam 2003). The Tin

and increasing RHin in the greenhouses (Al-Helal and AL-

shows that the Tin-s was lower than the Tin-c throughout

are significantly affected by the SRout (Abdel-Ghany et al. 2015). Therefore, shading mainly uses to control the Tin

the period of the experiment. This confirms that external

into the greenhouse. In addition to the effect of outside

evaporatively cooled greenhouses under hot climatic

climate, the RHin in arid region greenhouses depends on

conditions of arid regions. This result agrees with Al-Helal

the evapotranspiration from plants and humidification from

and Al-Musalam (2003). In Fig. 3-C, the high value of the

the cooling system (Ahmed et al. 2016).

RHin in the morning in both greenhouses (RHin-c and RHin-s)

shading has a significant effect on reducing the Tin in the

by reducing the amount of the transmitted solar radiation

is mainly due to the solar heating effects (after sunrise)

4.1. The Tin and RHin in the arid region’s greenhouses

that increase air temperature (Tin-c and Tin-s) and enhance

evaporation of dew drops and mist that may be condensed

Based on the obtained results from this study, the Tin was

at night when the air temperature was low. When the

RHout, respectively, as shown in Fig. 3-B and C. This is

started to evaporate and then the RHin decreased to 55%

SRout got higher (8:30), the Tin increased, the water vapor

lower and the RHin was always higher than the Tout and

3m

1m

B 36

34

34

32

32 Tin (°C)

Tin (°C)

A 36

30 28

30 28

24 22

22

C 90

D 90

80

80

70

70 RHin (%)

26 24

RHin (%)

26

60 50

60 50

40

40

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

30 06:00 08:00 10:00 12:00 14:00 16:00 18:00 Time

Fig. 8 Diurnal variation of the average values of air temperature (Tin) and relative humidity (RHin) measured on the vertical direction, in the shaded (Gs) and unshaded (Gc) greenhouses, during the period of the experiment (16th to 26th, Oct. 2017). A, the vertical variation of Tin-c. B, the vertical variation of Tin-s. C, the vertical variation of RHin-c. D, the vertical variation of RHin-s. Table 8 The vertical variation of the air temperature and relative humidity during the period of the experiment (16th to 26th, Oct. 2017) Variable Air temperature (°C) Relative humidity (%)

Measuring location 1-m height 3-m height 1-m height 3-m height

Unshaded greenhouse Mean±SD Min. Max. 28.5±1.6 b 24.0 30.7 32.0±3.5 a 23.7 35.9 58.5±9.6 a 50.0 84.6 48.9±13.8 b 37.6 84.3

Shaded greenhouse Mean±SD Min. 27.0±0.9 b 24.0 31.2±2.7 a 23.8 57.9±5.2 a 51.5 44.4±9.2 b 37.8

Different letters indicate significant differences between the air temperature and relative humidity averages at P=0.05.

Max. 28.2 33.7 73.7 72.3

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in both greenhouses (Jamaluddin et al. 2014). In addition, the reduction in the RHin-s compared to the RHin-c in the morning period because the external shading delays the operational time of the evaporative cooling system in the Gs which is considered the main source of water vapor in the greenhouse. In other words, the external shading reduced the Tin and made the operating time of the cooling system shorter than that in the Gc. This makes the water vapor content, as well as the possibility of mist and dewdrops condensation in the Gc higher than that in the Gs.

4.2. The horizontal distribution of the Tin and RHin along the pad-fans direction In greenhouses which are cooled using pad-fans cooling systems, exhausted fans are usually mounted on one side of the greenhouse. The fans pull the inside air and then the outside air flows through the cooling pad. As the air moves along the pad-fans direction, it heats up, and its temperature increases whereas its relative humidity decrease (Teitel et al. 2010). In arid climate greenhouses, the SRout as well as the direction of the air flow have a direct effect on the distribution of the Tin and RHin along the pad-fans direction. As expected, the Tin in both greenhouses tends to increase and the RHin tends to decrease away from the cooling pad toward the fans as shown in Figs. 4 and 5. The increasing of the Tin with increasing the distance away from the cooling pad comes as a result of the convection heat added to the inside air from the greenhouse components (cover, floor, structures, plants, etc.). The decreasing of the RHin was because the solar heat gain reduces the moisture content of the air inside the greenhouse. A similar result has been reported by Kittas et al. (2001) and Al-Helal (2007). Generally, the obtained results showed that the distribution of the Tin-s and the RHin-s was more uniform than the distribution of the Tin-c and RHin-c as shown in Tables 4 and 5. This result agrees with Kittas et al. (2003), Chen et al. (2003), Bartzanas and Kittas (2005), and Oz et al. (2009). The statistical analysis of the data (Tables 4 and 5) and observation of figures (Figs. 4 and 5) revealed that the distribution of the Tin and RHin at 3-m was less uniform than that at 1-m height in both greenhouses. This result can be explained as with the increase in height, the effect of SRout on increasing the Tin and decreasing the RHin increased. Using mechanical ventilation (pad-fans systems) usually enhances the air mixing and cooling performance in the pad-fans level; this performance decreases with height. Therefore, a limited effect for the external shading on improving the uniformity of the Tin and RHin distribution at 3-m height was found because the fans were located at 0.75 m over the greenhouse floor. This result agrees with Arbel et al. (2003) who reported that

the ventilation direction makes the horizontal distribution of the Tin and RHin more uniform along the greenhouse length than the greenhouse height.

4.3. The horizontal distribution of the Tin and RHin along the E–W direction In the N–S oriented greenhouses in sunny regions, the distribution of the Tin along the E–W direction is usually less uniform than the distribution along the pad-fans direction. The intensity of SRout and the orientation of the greenhouse are the most important factors affecting the spatial distribution of the Tin. The increasing of SRout intensity and the Tout led to increase the radiative-convective heat transfer from the outside environment into the greenhouse (Stanciu et al. 2016). On the other hand, the irregular distribution of the Tin along the greenhouse E–W direction increased with the increase of height above the greenhouse floor. Combining the external shading with the evaporative cooling system has a significant effect on improving the Tin uniformity distribution along the E–W direction. The orientation of the greenhouse has no effect on the distribution of the RHin in arid region’s greenhouses. The reason for this is that RHin mainly depends on the evaporative cooling performance and the evapotranspiration rate from plants. Other factors affecting the RHin in arid regions greenhouses are the intensity of SRout, Tout and RHout. The RHin-c and RHin-s distribution pattern was similar to the distribution pattern along the pad-fans direction. In a similar manner to the distribution of the RHin along the pad-fans direction at 3-m height, the distribution of the RHin along the E–W direction at 3-m height also increased. This result agrees with Zhao et al. (2001). In this study, we conclude that the distribution of the Tin along the E–W direction is the first microclimate parameter that should be improved in the arid climate greenhouses by improving the cooling and ventilation performance.

4.4. The vertical distribution of the Tin and RHin The increasing of the Tin and decreasing of the RHin with height is a natural thermal gradient in the greenhouse environment. The obtained results in Fig. 8 showed that the Tin and RHin in the vertical direction varied significantly with the intensity of the SRout. This is because of the convection heat transfer from the lower level to the upper level. As the hot air moves to a higher altitude, it gets more energy and less density. This result agrees with Zhao et al. (2001) who reported that Tin increases and RHin decreases with the increase of height above the greenhouse floor. In this study, the vertical distribution of the Tin and RHin is

Hesham A. Ahmed et al. Journal of Integrative Agriculture 2019, 18(12): 2869–2882

the second microclimatic parameter should be improved in arid climate greenhouses by improving the cooling and ventilation performance.

5. Conclusion In this study, the spatial distributions of the air temperature (Tin) and relative humidity (RHin) in arid regions greenhouses were evaluated. Two evaporatively-cooled greenhouses with and without external shading were used. The evaluation included a description of the Tin and RHin distribution along the greenhouse length (pad-fans direction), width (eastwest direction), and height (1-m and 3-m height above the greenhouse floor). The following conclusions can be summarized: (1) In arid climate regions, the incident solar radiation has a major effect on the spatial distribution of the Tin and RHin. The highest spatial variation in the Tin and RHin occurred along with a peak in solar radiation at around noon time. (2) The external shading reduced the spatial distribution of the Tin and RHin, particularly along the length of the greenhouse due to reducing the effect of solar radiation and reducing the thermal radiation emission inside the greenhouse. Therefore, the cooling efficiency of the padfans cooling system was improved by 12%. (3) The distribution of the Tin and RHin in the vertical direction and the distribution of the Tin along the greenhouse sidewalls were observed to be much higher than the distribution along the pad-fans direction. (4) The vertical distribution of the Tin and RHin is crucial, especially for the high crops. In future studies, more uniformity of the Tin and RHin are expected by combining the external shading with air mixing to achieve less solar radiation transmission and prevent heat accumulation.

Acknowledgements The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University, (RG-1435-074), King Saud.

References Abdel-Ghany A M, Al-Helal I M. 2011. Analysis of solar radiation transfer: A method to estimate the porosity of a plastic shading net. Energy Conversion and Management, 52, 1755–1762. Abdel-Ghany A M, Picuno P, Al-Helal I, Alsadon A, Ibrahim A, Shady M. 2015. Radiometric characterization, solar and thermal radiation in a greenhouse as affected by shading configuration in an arid climate. Energies, 8, 13928–13937.

2881

Ahemd H A, Al-Faraj A A, Abdel-Ghany A M. 2016. Shading greenhouses to improve the microclimate, energy, and water saving in hot regions: A review. Scientia Horticulturae, 201, 36–45. Al-Helal I M. 2007. Effects of ventilation rate on the environment of a fan-pad evaporatively cooled, shaded greenhouse in extreme arid climates. Applied Engineering in Agriculture, 23, 221–230. Al-Helal I M, Al-Musalam I M. 2003. Influence of shading on the performance of a greenhouse evaporative cooling system. Arab Gulf Journal of Scientific Research, 21, 71–78. Arbel A, Barak M, Shklyar A. 2003. Combination of forced ventilation and fogging systems for cooling greenhouses. Biosystems Engineering, 84, 45–55. Arbel A, Yekutieli O, Barak M. 1999. Performance of a fog system for cooling of greenhouses. Journal of Agricultural Engineering Research, 72, 129–136. Bailey B J. 1991. The environment in evaporatively cooled greenhouses. Acta Horticulture, 287, 59–66. Bartzanas T H, Kittas C. 2005. Heat and mass transfer in a large evaporative cooled greenhouse equipped with a progressive shading. Acta Horticulture, 691, 625–632. Bojaca C R, Gil R, Cooman A. 2009a. Use of geostatistical and crop growth modeling to assess the variability of greenhouse tomato yield caused by spatial temperature variations. Computer and Electronics in Agriculture, 65, 219–227. Bojaca C R, Gil R, Gomez S, Cooman A, Schrevens E. 2009b. Analysis of greenhouse air temperature distribution using geostatistical methods. Transactions of the ASABE, 52, 957–968. Bucklin R A, Henley R W, McConnell D B. 1993. Fan and Pad Greenhouse Evaporative Cooling Systems. Circular 1135: December 1993. Florida Cooperative Extension Service, University of Florida, Gainesville, FL, USA. Chen C. 2003. Prediction of longitudinal variations in temperature and relative humidity for evaporative cooling greenhouses. Agricultural Engineering Journal, 12, 143–164. Chen C, Shen T, Weng Y. 2011. Simple model to study the effect of temperature on the greenhouse with shading nets. African Journal of Biotechnology, 10, 5001–5014. Ganguly A, Ghosh S. 2011. A review of ventilation and cooling technologies in agricultural greenhouse application. Iranica Journal of Energy and Environment, 2, 32–46. Jamaludin D, Ahmad D, Kamaruddin R, Jaafar H Z E. 2014. Microclimate inside a tropical greenhouse equipped with evaporative cooling pads. Pakistan Journal of Science and Technology, 22, 255–271. Jaramillo M E, Vera-Romero I, Martínez-Reyes J, Ortíz-Soriano A, Barajas-Ledes E. 2014. Empirical model to calculate the thermodynamic wet-bulb temperature of moist air. Engineering, 6, 500–506. Kittas C, Bartzanas T, Jaffrin A. 2001. Greenhouse evaporative cooling: Measurement and data analysis. Transactions of the ASABE, 44, 683–689. Kittas C, Bartzanas T, Jaffrin A. 2003. Temperature gradients in a partially shaded large greenhouse equipped with

2882

Hesham A. Ahmed et al. Journal of Integrative Agriculture 2019, 18(12): 2869–2882

evaporative cooling pads. Biosystem Engineering, 85, 87–94. Kumar K S, Tiwari K N, Jha M K. 2009. Design and technology for greenhouse cooling in tropical and subtropical regions: A review. Energy and Buildings, 41, 1269–1275. Li S, Willits D. 2008. Modeling thermal stratification in fan ventilated greenhouses. Transactions of the ASABE, 51, 1735–1746. Oz H, Atilgan A, Buyuktas A, Alagoz T. 2009. The efficiency of fan-pad cooling system in greenhouse and building up of internal greenhouse temperature map. African Journal of Biotechnology, 8, 5436–5444. Van Pee M, Janssens K, Berckmans D, Lemeur R. 1998. Dynamic measurement and modeling of climate gradients around a plant for micro-environment control. Acta Horticulture, 456, 399–406. Ryu M J, Ryu D K, Chung S O, Hur Y K, Hur S O, Hong S J, Sung J H, Kim H H. 2014. Spatial, vertical, and temporal variability of ambient environments in strawberry and tomato greenhouses in winter. Journal of Biosystem Engineering, 39, 47–56. Salokhe V M, Soni P, Tantau H J. 2005a. Effect of net size on horizontal temperature gradients in naturally-ventilated tropical greenhouses. Journal of Food, Agriculture and Environment, 3, 316–322. Salokhe V M, Soni P, Tantau H J. 2005b. Spatial variability of air humidity inside naturally ventilated tropical greenhouse. International Agrophysics, 19, 329–336. Sapounas A A, Nikita-Martzopoulou C, Bartzanas T, Kittas C. 2008a. Fan and pad evaporative cooling system for

greenhouses: Evaluation of a numerical and analytical Model. Acta Horticulture, 797, 131–138. Sapounas A A, Nikita-Martzopoulou C, Spiridis A. 2008b. Prediction the spatial air temperature distribution of an experimental greenhouse using geostatistical methods. Acta Horticulture, 801, 495–500. Seginer I. 1994. Transpirational cooling of a greenhouse crop with partial ground cover. Agriculture and Forest Meteorology, 71, 265–281. Sethi V P, Sharma S K. 2007. Survey of cooling technologies for worldwide agricultural greenhouse applications. Solar Energy, 81, 1447–1459. Stanciua C, Stanciua D, Dobrovicescua A. 2016. Effect of greenhouse orientation with respect to E-W axis on its required heating and cooling loads. Energy Procedia, 85, 498–504. Teitel M. 2001. The effect of insect proof screens in roof openings on greenhouse microclimate. Agricultural and Forest Meteorology, 110, 13–25. Teitel M, Atias M, Barak M. 2010. Gradients of temperature, humidity, and CO2 along a fan-ventilated greenhouse. Biosystem Engineering, 106, 166–174. Willits D H, Peet M M. 2000. Intermittent application of water to an externally mounted greenhouse shade cloth to modify cooling performance. Transactions of the ASABE, 43, 1247–1252. Zhao Y, Teitel M, Barak M. 2001. Vertical temperature and humidity gradients in a naturally ventilated greenhouse. Journal of Agricultural Engineering Research, 78, 431–436.

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