Prediction of the Effect of Insect-proof Screens on Climate in a Naturally
Ventilated Greenhouse in Humid Tropical Climates
P.O. Ajwang
Department of Biomechanical and
Environmental Engineering
Jomo Kenyatta University of Agriculture
and Technology
P.O. Box 62000, 00200-Nairobi
Kenya
H-J. Tantau
Institute of Horticultural and Biosystems
Engineering
University of Hannover
Herrenhaeuser Str. 2
D-30419, Germany
Keywords: Discharge coefficient, greenhouse climate, energy and mass balance
Abstract
The impact of different types of insect-proof screens on greenhouse climate in
humid tropics needs to be quantified in order to develop technically and economically
optimal greenhouse constructions. Thus, physical properties of various insect-proof
screens were determined in the laboratory at the Institute of Horticultural and
Agricultural Engineering, University of Hannover, Germany. Specially adapted experimental greenhouses were built at AIT campus in Bangkok, Thailand. A dynamic
energy and mass balance model of the greenhouse system was developed to predict the
greenhouse climate from external weather data and properties of insect screens.
Scenario simulations were carried out to predict the effect of screen properties on
greenhouse climate. External and greenhouse climate measurements were made in
Bangkok. The internal climate measurements were carried out concurrently in two
similar, naturally-ventilated greenhouses covered with different insect-proof screens
on ventilation openings. Tomato (Lycopersicon esculentum ‘King Kong II’) plants were
grown in the greenhouse during the experimental period. Model predictions of greenhouse air temperature were compared to measurements from the two greenhouses and
good agreement was achieved. The results form a good basis for decisions on screened
greenhouse design improvements and climate control interventions in screened greenhouses in the humid tropical climates.
INTRODUCTION
Although temperatures and global radiation are very suitable for vegetable
cultivation throughout the year in the humid tropics, open air cultivation is hindered by
heavy rainfall and high humidity which cause crop damage and infestation by diseases.
High incidence of pests such as thrips and aphids is also a big problem. Therefore,
greenhouse systems which can provide the optimal plant growth environment while
offering maximum protection against pests need to be developed. According to von
Zabeltitz (1993), air temperature and air humidity should not be higher in tropical
greenhouses than in the open air.
Cooling-related expense is one of the most important greenhouse operation costs
in the humid tropics. But the efficiency of fan and pad cooling systems in greenhouses in
the humid tropics is limited due to high humidity of ambient air (H-J. Tantau, pers.
commun., 2004). It is also expensive. Naturally-ventilated greenhouses can result in
saving of cooling-related costs. Physical exclusion of pests through the use of insect-proof
screens on greenhouse ventilation openings is an increasingly popular way of reducing
pesticide applications. A consequence of the use of insect-proof screens is that they
restrict air-flow and thus affect the climate in the greenhouse. The effect of insect-proof
screens on greenhouse climate depends on the physical properties of the screens (i.e.
discharge coefficient and spectral transmissivity) and the ratio of the screen area to floor
area.
Different authors have reported different values of discharge coefficients of
greenhouse openings (Sase and Christianson, 1990; Montero et. al. 1997; Munoz et. al.
Proc. IC on Greensys
Eds.: G. van Straten et al.
Acta Hort. 691, ISHS 2005
449
1999; Mears and Both 2001). These differences perhaps indicate the variety of
greenhouse configurations that have been studied. Simulation studies have shown that
greenhouses with insect-proof screens placed on side and roof ventilation attain higher air
temperatures compared to greenhouses with no screens on the openings (Teitel, 2001 and
Fatnassi et al., 2003). However, no study has been reported on the use of energy and mass
balance approach to predict climate in screened greenhouses with crops in the humid
tropics. In this study we sought to identify which insect-proof screens could meet the twin
goals of physical exclusion of pests and adequate natural ventilation, in the specially
adapted greenhouses for the humid tropics.
MATERIALS AND METHODS
Experimental Site
Field experiments were carried out at a purposely-built greenhouse complex at the
Asian Institute of Technology (AIT) which is located approximately 42 km to the north of
Bangkok in Thailand. The 200 m² greenhouse (fig. 1) had surface area to floor area ratio
of 2.25. It was fitted with ant-insect screen on the ventilation openings, i.e. on both the
wall and roof openings. The total area of the screened ventilation openings was 228 m2.
The rest of the surface area of the greenhouse was covered with polyethylene film.
Tomato plants were grown in pots placed at 1.60 m spacing (between rows). A
drip fertigation system was installed to supply water to the plants. Automatic control of
the drip fertigation system was achieved through the use of solenoid valves. Irrigation
control was based on radiation sum.
Experimental Measurements
Light transmission of the nets and plastic films was determined using a
photometer. The dimensions of the screen holes were adopted from measurements by
Klose (2002). In order to calculate the discharge coefficients of different screen materials,
airflow and pressure drop measurements were carried out using a wind tunnel set-up.
Discharge coefficients were determined by relating the pressure drop to airflow using the
Bernoulli equation as outlined by Ajwang et al. (2002).
Greenhouse air temperature and relative humidity measurements were achieved
using aspirated psychrometers. Two psychrometers were installed in each greenhouse at a
height of 1.5 m above ground level. Leaf temperature was estimated using the regression
model by Wang and Deltour (1999). Global radiation was measured using solarimeters
from Kipp and Zonen Ltd, Delft, Netherlands. For outside global radiation measurement,
the solarimeter was positioned at 1 m height. In the greenhouse, the solarimeters were
positioned above the plant canopy on the rails. Wind speed measurements were made
using a cup anemometer with a measuring range of 0.5-50 m/s from Thies Klima GmbH,
Germany. The instrument was mounted on 8 m high mast next to the greenhouse complex
in an area free of obstructions. All the instruments were connected to MCU-ITG 1996
data logger developed by the Institute of Horticultural and Agricultural Engineering of the
University of Hannover. The data loggers in the greenhouses were connected to one
central computer via a RS-485 bus cable. All the data from the sensors were sampled at
15 second intervals and mean values computed at 30-minute intervals were recorded on
the computer.
Data Analysis
A coupled energy and mass balance model of the greenhouse system was built
using the Matlab/Simulink toolbox (Mathworks, 1986). Greenhouse ventilation rate was
calculated according to Kittas et al. (1997).The wind effect coefficient was assumed to be
0.09 based on literature (Boulard and Baille, 1995; Baptista et al., 1999) Other equations
for the model were obtained from standard greenhouse engineering literature The model
uses four weather parameters as input variables namely; wind speed, global solar
radiation, outside relative humidity and outside air temperature. The effect of the changes
450
in several parameters including the discharge coefficient of screens, leaf area index, and
ratio of vents to screen area could be examined using the model. The model outputs
include predicted greenhouse air temperature, relative humidity, evapo-transpiration rate,
vapour pressure deficit and ventilation rate.
Using weather data obtained from the AIT campus in Bangkok between 10th -14th
August, 2003, scenario simulations were carried out to examine the effect of discharge
coefficient on ventilation rate, temperature and vapour pressure deficit (VPD). It was
assumed that the greenhouse had a tomato crop of leaf area index 4. The total opening of
the screened area was taken to be 228 m² (same as the actual total screened area on the
experimental greenhouse).
Data for validation of the energy and mass balance model was obtained from the
experimental site during the period between 15th and 29th August, 2003. Measurements
were made concurrently in two greenhouses covered with screens of different mesh 78x52 and 40x38 (abbreviated as 78- and 40-mesh respectively in the following
paragraphs). Tomato plants were planted in both greenhouses on 10th August 2003 such
that leaf area index (LAI) of plants in the two greenhouses were the same during the
experimental period. The values of LAI for different growth stages were adopted from the
work of Kleinhenz (2003).
RESULTS AND DISCUSSION
Effect of Discharge Coefficient
Table 1 shows a summary of the properties of the insect-proof screens that were
determined. Screens with small hole sizes had the smallest discharge coefficients. The
lowest discharge coefficient was that of Econet S which is an anti-thrips screen while
Econet B which has the largest hole size has the largest discharge coefficient also.
Figure 2 illustrates the effect of discharge coefficient on greenhouse ventilation
rate. Discharge coefficients above 0.2 generally provided ventilation rates above 0.75
volume changes per minute and can be considered adequate. However, the ventilation rate
is strongly dependent on wind speed as is implicit from the fluctuations on the graph.
Discharge coefficients of 0.3 and above resulted in ventilation rates of about 1 volume
change per minute and above. But given that the nets with discharge coefficients of 0.3
and above are likely to be penetrated by smaller insects like thrips it would not be safe to
use them under field conditions. Therefore, to provide adequate ventilation for cooling
while preventing the entry of harmful insects into the greenhouse, the discharge
coefficients of the openings should be between 0.2 to 0.3.
The effects of discharge coefficient on greenhouse air temperature is illustrated in
Figure 3. The lowest temperatures are predicted for the highest discharge coefficient i.e.
0.5. On the other hand, when the discharge coefficient is 0.01, the predicted temperature
rises significantly depending on the external weather conditions, principally solar
radiation and wind speed. A difference in temperature of about 10 oC can be realized
between a greenhouse covered with a net of 0.5 discharge coefficient as opposed to a
closed greenhouse (discharge coefficient 0.01). Larger discharge coefficients in the range
of 0.2-0.5 results in small temperature increases i.e. below 1 oC.
The predicted vapour pressure deficit in the greenhouse increased with increase in
temperature. The simulation results suggest that closed greenhouses will have higher
vapour pressure deficit than open greenhouses during periods of high solar radiation.
Model Validation
In Fig. 4 it can be observed that the air temperature in the two greenhouses was
generally higher than the outside air temperature. Larger differences between the
greenhouse air temperature and the outside air temperature is evident during the day.
During some periods the difference between inside and outside air temperatures was as
high as 5 oC. In the evenings and at night, the recorded inside and outside air temperatures
were comparable. The temperatures recorded in 78-mesh greenhouse were generally
451
higher than in the 40-mesh greenhouse during periods of high irradiance. During night
time, the differences in temperature between the two greenhouses were minor. Larger
differences in temperature were registered during the day time, with the highest difference
of about 3 oC between the two greenhouses.
Model predictions of the air temperature in the 78-mesh greenhouse were in good
agreement with the measured values (Fig. 5). Differences between the predicted and
measured values were more pronounced during the day-time. Over-predictions and underprediction of temperatures are evident from the figure suggesting the randomness of
distribution of the simulation errors. Both the measured and predicted temperatures in the
78-mesh greenhouse were marginally higher than those in the 40-mesh greenhouse. The
result suggests that the discharge coefficients of 0.22 for 78-mesh greenhouse and 0.32
for the 40-mesh greenhouse satisfactorily describe the restriction of air flow in the two
greenhouses.
In the 78-mesh greenhouse the vapour pressure profiles predicted were higher than
in the 40-mesh greenhouse. The higher vapour pressure deficit in 78-mesh greenhouse is
attributable to higher temperature in the greenhouse. Higher values of vapour pressure
deficit are predicted when plants have small LAI than when the LAI is large. In the day
time, the vapour pressure deficit was generally above 2 kPa in both greenhouses.
CONCLUSIONS
The most notable effect of the anti-insect screens is on greenhouse air temperature.
The study has established that the ambient temperature in the Bangkok region is generally
above 24 oC during day-time and occasionally fell below this value during the night.
Ambient temperatures often exceed 30oC during the day for most periods of the year.
With the greenhouse design used in the study and assuming a young tomato crop, the
results showed that the use of an anti-thrips net with a discharge coefficient 0.22 can
cause a temperature increase of up to 5 oC relative to the ambient, depending on the
external climatic conditions. Theoretical simulations using different areas of leaf area
index showed that a 2 oC reduction in greenhouse air temperature can be attributed to a
full tomato canopy. Thus the increase in temperature in an anti-thrips greenhouse could be
as high as only 3 oC after considering the evapo-transpirational cooling effect of a full
tomato crop. Generally, tomatoes grow best in temperatures ranging from 20 to 27 oC
(Hanson et al., 2000). Fruit setting is poor when average temperatures exceed 30 oC or
fall below 10 oC. It can thus be concluded that the adapted greenhouse investigated in this
study would require cooling in order to make tomato production efficient. Strategies of
reducing the greenhouse air temperature such as the use of white mulches and growing
pots, and use of near infrared (NIR)-absorbing plastics should be explored.
ACKNOWLEDGEMENTS
This study was conducted using a grant from the German Research Council
(DFG). The authors are very grateful for the generous support.
Literature Cited
Ajwang, P., Tantau, H.-J. and von Zabeltitz, C. 2002. Insect screens for integrated
production and protection in horticulture: a review of the physical and technical
basics. Gartenbauwissenschaften (now European Journal of Horticultural Sciences)
67(2): 45- 49.
Baptista, F.J., Bailey, B.J., Randall, J.M. and Meneses, J.F. (1999). Greenhouse
ventilation rate: theory and measurement with tracer gas techniques. Journal of
Agricultural Engineering Research, 72, 363-374.
Boulard, T. and Baille, A. 1995. Modelling of air exchange rate in a greenhouse equipped
with continuous roof vents. Journal of Agricultural Engineering Research, 61, 37-48.
Fatnassi, H., Boulard, T. and Boiurden, L. 2003. Simulation of climatic conditions in fullscale greenhouse fitted with insect-proof screens. Agricultural and Forest Meteorology
118 (2003) 97-111.
452
Hanson, P., Chen, J.T., Kuo, C.G., Morris, R. and Opena, R.T. 2000. Suggested cultural
practices for tomato. International cooperators’ guide. AVRDC pub no. 00508.Taiwan.
Kittas, C., Boulard, T. and Papadakis, G. 1997. Natural ventilation of a greenhouse with
ridge and side opening: sensitivity to temperature and wind effects. Trans. ASAE 40
(2), 415 – 425.
Kleinhenz, V. 2003. Greenhouse tomato leaf development. Research progress report.
Protected Cultivation Project, Asian Institute of Technology / University of Hannover.
Unpublished.
Klose, F. 2002. Investigations of insect-proof screens – measurements and evaluation of
air and spectral transmissivities (in German). MSc. thesis, Institute of Horticultural
and Agricultural Engineering, University of Hannover.
Mathworks (1991). Matlab User’s Guide. The Mathworks Inc., South Natick, MA 01760,
USA
Mears, D.R. and Both, A.J. 2001. Insect screening and positive pressure ventilation from
tropical and subtropical greenhouse facilities. Proceedings of the international
symposium on design and environmental control of tropical and subtropical
greenhouses. April 15-18, 2001. National Taiwan University, Taichung, Taiwan.
Montero, J.I., Munoz, P. and Anton, A. 1997. Discharge coefficients of greenhouse
windows with insect-proof screens. Acta Hort. Vol. 443, 71-77.
Munoz, P., Montero, J.I., Anton, A. and Giuffrida, F. 1999. Effect of insect-proof screens
and roof openings on greenhouse ventilation. Journal of Agricultural Engineering
Research 73, 171-178.
Wang S. and Deltour, J. 1999. An experimental model of leaf temperature for greenhousegrown tomato. Proc. of the Int. Symposium on Greenhouse Management for Better
Yield and Quality in Mild Winter Climates. Acta Hort. 486, ISHS 1999.
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State Univ. July 29 - Aug 1, 1990.
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Rome.
453
Tables
Table 1. Physical properties of insect-proof screens.
Material
*No-Screen
Econet S
Supplier
Hole-Size
(mm²)
% Light
Transmission
Discharge
Coefficient
%
Ventilation
reduction
70
Ludvig
Svensson
0.0196
85
0.5
0.15
Econet F
Ludvig
Svensson
0.351
88
0.36
28
Econet M
Ludvig
Svensson
0.18
89
0.31
38
Econet T
Ludvig
Svensson
0.0525
86
0.22
56
Econet SF
Ludvig
Svensson
0.2184
85
0.28
44
Econet L
Ludvig
Svensson
0.36
79
0.36
28
Econet B
Ludvig
Svensson
4
87
0.48
4
Brinkmann T
0.0682
77
0.23
54
0.2079
88
0.31
38
0.9975
88
0.47
6
Polysack
Anti-Virus
Howitec
BV
Howitec
BV
Howitec
BV
Polysack
RACS
0.2184
80
0.33
34
Polysack
Bionet
Polysack
RACS
0.2132
67
0.33
34
Brinkmann W
Brinkmann M
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Figurese
0.8m
3.7m
FANS
5m
4m
4m
4m
4m
4m
0.5m
5m
PLASTC FILM
INSECT-PROOF SCREEN
DOOR
Fig. 1. Sketch of the screened experimental greenhouse.
7
6
Air Changes (volumes/minute)
5
4
Cd=0.1
Cd=0.2
Cd=0.3
3
2
1
0
10-Aug
11-Aug
12-Aug
13-Aug
14-Aug
Time (Day)
Fig. 2. Effect of discharge coefficient on predicted ventilation rate.
455
Cd=0.01
Cd=0.2
Cd=0.5
Cd=0.1
Air temperature (°C)
45
40
35
30
25
20
10-Aug
11-Aug
12-Aug
13-Aug
14-Aug
T ime (Day)
Fig. 3. Effect of discharge coefficient on predicted greenhouse air temperature.
Temperature (°C)
40
Ti-meas78
Ti-meas40
To-meas
35
30
25
20
15Aug
16Aug
17Aug
18Aug
19Aug
20Aug
21Aug
22Aug
23Aug
24Aug
25Aug
26Aug
27Aug
28Aug
29Aug
29Aug
Time (Day)
Fig. 4. Comparison of measured outside and greenhouse air temperatures.
Time (Day)
Temperature (°C)
15-Aug
40
35
30
25
20
15
10
5
0
-5
16-Aug
17-Aug
18-Aug
19-Aug
Ti-pred
Ti-meas
dev
Fig. 5. Predicted (Ti-pred) vs. measured (Ti-meas) air temperature for 78-mesh
greenhouse. dev= difference between predicted and measured temperature.
456