Download NATURAL VENTILATION OF GREENHOUSES IN DESERT

NATURAL VENTILATION OF GREENHOUSES IN DESERT
CLIMATE
A,. Arbel, I. Segal, 0. Yekutieli, N. Zamir
Institute of Agricultural Engineering,
Agricultural Research Organization,
Bet Dagan,
Israel.
Abstract
Desert weather is usually characterized by clear skys, very
low precipitation and low air humidity. These conditions are
favorable
for greenhouse horticulture to produce winter
cropping. Development of greenhouses during recent years in
these regions in Israel is very rapid. Development started
with utilizing low and high tunnels, the latter showing
significant horticultural advantages.
The Institute of Agricultural Engineering was involved in
developing greenhouses to assist in the specific needs of the
desert areas in achieving the following:
1. Low cost of construction and maintenance.
2. Simple construction and covering procedures.
3. Large continuous growing space.
The result is a no-gutfeer, roof ventilated
('French
system),
5000m2
square
structure. Ventilation
of this
structure is achieved by opening the four sides and 'wedges'
in the roof (total 15)1 of horizontal area).
Theoretical considerations taking into account natural
ventilation and dry weather, forecasted acceptable1 climate
inside the greenhouse during winter
and
spring
days.
Comparison of temperature and humidity in the low structure
with a conventional (30 m length,- f| m gutter height with
openings on two sides and gables) showed small differences in
temperature while relative humidity in the low greenhouse was
higher - closer to required humidity.
Climate and horticultural experiences of the first year
will be presented with description of essential structures and
ventilation details.
Acta Horticulturae 281, 1990
Greenhouse Construction, Design
Introduction
Climate in desert regions in Israel (specifically the Arava
region) is specified by very low precipitation and high solar
radiation (less than 60 mm - about 10-15 cloudy days per
year). Relative
humidity is very low and daily ambient
temperatures
are
relatively high (Figs. 1,2). These are
favourable conditions for growing horticultural products for
export during winter. The development of greenhouses in desert
areas is one of the major efforts of Israeli agriculture. The
Institute
of
Agricultural
Engineering
has
initiated
development of low cost wide span low silhouette gutterless
greenhouse for desert climate.
These
greenhouses were designed with the purpose
achieving:
1) Low cost of construction.
2) Minimum maintenance requirements and simple procedures
replace covering films.
3) Large continuous growing space.
of
to
The commercial version is a 70x70m greenhouse with a
minimum height of 1.8-2.2m. Ventilation is achieved by opening
edges in the roof (so called 'French' system) and curtains in
the four sides. Total openings are about 15% of floor area.
One of the main forseeable disadvantages of the new type,
low wide span (LWS)
greenhouse is natural
ventilation
ability. However, preliminary calculations taking into account
the large contribution of evaporative cooling due to plant
transpiration in dry climates promised acceptable climate
conditions during autumn and spring. The use of conventional
and non-conventional evaporative cooling systems is also very
promising in dry climates. Direct spraying or fogging could be
the solution
for cooling of low wide span (LSW) greenhouses
when
vegetation
is not a sufficient source for water
evaporation.
Investigation of the climate conditions in greenhouses in
the Arava desert region is described in the following paper.
Methods and Materials
During the first year of investigation manual measurement
of climate conditions (temperature, relative humidity, wind
velocity) was carried out during mid-day hours
(11-14)
starting 30 November 1987 up to 8 May 1988. These measurements
were carried out in 17 different commercial greenhouses in the
Arava region. These 17 greenhouses can be represented by four
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groups: (Fig. 7.)
1) Walk in tunnels (almost semi-circle- Gl). Width 8.5 m,
height 3.5 m, side curtains and end opening. Total openings
25?! of floor area.
2) Low Wide Span Greenhouse (-G12)
Gutterless structure roof ventilated (French system) operated
centrally by cables on pipes.
Minimum height 1.8 m, maximum 3.0 m, 7.12 m span, total area
5,000 m2 (70x70m approx). Max. opening 5% of floor area (5% on
roof).
3) Regular Plastic Covered Greenhouse (-G3)
140x32 m, gutter height 4 m, span 7.5m, curtains 2.8m height
on all four sides, total opening area 21%.
k) Similar regular greenhouse (- C4) 99x4iim with roof
(5%) and side curtains (18J! of floor area).
Most measurements
hygrometer (Hygroskop
opening
were made by a capacitance type ThermoGT2 by Rotronics).
The readings of this instrument were compared with a
conventional aspirated psychrometer by Kasela (U.K.). Wind
velocity was measured by PA 4000 by Digital. Radiation was
measured occasionally. However, on most relevent days sky was
clear and radiation intensity is very close to "clear sky
radiation" calculated by standard methods (Duffie & Beckman,
Fig. 1).
Results. Discussion and Conclusions
Temperatures and relative humidity in the low wide span
greenhouse during seasons are given in figures 3 and 4. Note
that the lines do not represent continuous
temperature
measurements, but connect points taken every two weeks.
Figures 5 and 6 show the differences betwen four major types
of greenhouses and the ambient.
During the beginning of the season higher temperature
differences were achieved, due to partial opening of the
greenhouses.
Towards the end of the season with well
developed plants (trained tomatoes) inside temperatures were
sometimes lower than those outside
due
to
effective
evaporative cooling.
Ventilation and cooling in most greenhouses during all
seasons, including spring months, was sufficient. Temperatures
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inside the greenhouses were usually not higher than required
(Fig. 3). Differences between the various types were not
large (Fig. 5). Evaporative cooling due to plant transpiration
during spring months explains the relatively low temperatures
inside the greenhouse. Humidity levels (Fig. k) in the LWS
greenhouses were acceptable. The low humidity of the outside
air (20-30%) during all seasons explains these acceptable
conditions in the greenhouses.
The Low Wide Span Greenhouse was more humid than the
others. However, humidity levels were too low in the other
greenhouses due to over ventilation.
The following conclusions and recommendations can be drawn
from this climate survey:
1) Acceptable conditions for growing can be achieved in all
greenhouses by natural ventilation in the arid Arava region.
The low wide span greenhouse can be used without causing overheating even during autumn and spring.
2) More investigations concentrating on the control of
openings should be carried out.
Also the operation of spray and fog type evaporative cooling
for arid zones when plant leaf area is too small should be
further investigated.
3) The advantage of arid zones enables growing during winter
due to stronger solar radiation but also enables simple
cooling by natural ventilation and evaporation in relatively
simple low and inexpensive structures.
References
1) Duffie, J.A. and Beckman, W.A. 1980.
Solar Engineering of Thermal Processes.
John Wiley & Sons, Inc. U.S.A.
2) Arbel, A., Zamir, N., Yekutieli, 0. and
Meron, I. 1988. Mid-day Climatic
Conditions in the Central Arava WinterSpring 1987/88. Internal Report (in
Hebrew).
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11 4
B
0
>
8
7
,
S
10
,
11
,
le
,
1
,
e
3
MONTH
Fig. 1.
Global Solar Energy in I s r a e l
Month
0
T.mox
T.min
O
R.H.14.
A
R.H.OB
F i g . 2. Temperatures and r e l a t i v e humidities in
Arava region
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sa
30
29
20
27
26
23
22
21
20
2B.2
out
+
mean
14.3
27.3
11.4
25.4
DATE
1
6
Fig. 3 . Air temp, in the LWS greenhouse and ambient ( o u t )
l o c a t i o n s 1 , 2 , 3 , 4 a r e a t height 1.2m evenly
d i s t r i b u t e d along the greenhouse (North— South)
DATE
•
out
4
mean
»
1
6
2
X
3
7
4.
Fig. 4 . R e l a t i v e humidity i n the LWS greenhouse and ambient.
Locations 1, 2 , 3 , 4 see Fig. 3 .
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•
03
4- 04.
DATE
«
012
h <31
F i g . 5 . Temperature d i f f e r e n c e s (compared with ambient) of t h e
f o u r major types of greenhouses.
G3 - r e g u l a r greenhouse, G4 r e g u l a r with roof opening,
G12 - LWS greenhouse, G1 - walk-in tunnel
DATE
D
G.i
+
G4.
6
G12
A
G1
F i g . 6 . R e l a t i v e humidity d i f f e r e n c e s compared with ambient
of the f o u r major greenhouses ( F i g . 5 ) .
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