DESIGN OF GREENHOUSE NATURAL VENTILATION SYSTEMS(I)
E. Baeza, J.C. Lopez, J.I. Montero
EUPHOROS PROJECT
WORKSHOP SICILY (RAGUSA)
OCTOBER 6 2011
Mediterranean greenhouses
use plastic films as cladding and
investment is moderate, climate
control limited to natural ventilation
and shading (whitening)
The energy crisis caused the displacement of horticultural
production to the Mediterranean countries
Northern glasshouses are
sophisticated and provide almost
optimal conditions for plants all year
round
INTRODUCTION
MAIN GREENHOUSE TYPES IN THE MEDITERRANEAN BASIN
LOCAL TYPE GREENHOUSES
Low cost structures with little climate control besides
natural ventilation; they are built with local materials
(i.e. wood) and covered with polyethylene plastic
film. The parral-type greenhouse is probably the
most extended example of this type of structures in
terms of surface
Important problems associated to its design, such as the lack of tightness, low
radiation transmission in winter, et cetera, but perhaps its main drawback is the lack of
natural ventilation which is mainly due to three reasons:
•Low ventilation area, which is a result of a bad combination of side and roof ventilation and to
the construction of small roof vents due to the grower’s fear of sudden strong winds, as the
automation is really scarce.
•Inefficient ventilator designs: for roof ventilation flap ventilators are always preferable to rolling
ventilators since they provide larger ventilator rates at equal size (almost 3 times larger air flow
according to Pérez Parra et al., 2004).
•Use of low porosity insect screens. As discussed hereafter, insect-proof screens strongly
reduce the air exchange rate.
Computer simulations show that during the winter, increasing the roof slope from
11º to 45 º can increase daily light transmission by nearly 10% (Castilla, 2005) In
practice it is more useful to find a compromise between good light transmission
and construction cost, so most of the new greenhouses have 25-30º of roof slope.
MAIN GREENHOUSE TYPES IN THE MEDITERRANEAN BASIN
MAIN GREENHOUSE TYPES IN THE MEDITERRANEAN BASIN
PLASTIC COVERED INDUSTRIAL TPE GREENHOUSES
Multi tunnels are more hermetic than the parral type greenhouses and easier to equip with
cooling, heating and/or computer control.
In general, this group includes greenhouses which usually have more efficient ventilation
systems.
Condensation can occur in the upper inner part of the roof, so dripping is likely to occur
during humid and cold weather. Attempts have been made to solve this problem by
increasing the roof slope so that the arches are pointed-shape instead of circular shape,
but this has not totally eliminated condensation. On the other side for large span
greenhouses with insect-proof netting ventilation is insufficient, a subject discussed
hereafter.
MAIN GREENHOUSE TYPES IN THE MEDITERRANEAN BASIN
GLASSHOUSES
If glasshouses are to be constructed in climate areas warmer than Northern Europe,
especial attention should be paid to the improvement of ventilation; it is particularly
important to install sidewall vents and continuous roof vents to increase the ventilator area
when insect proof screens are a necessity. As discussed later, the combination of roof and
sidewall ventilation ensures larger ventilation rates, both under windy conditions (Kacira et
al., 2004) and especially, under low or zero wind conditions with buoyancy driven natural
ventilation (Baeza et al., 2009)
NATURAL VENTILATION
In mild winter climate areas, natural ventilation is
essential in greenhouse cultivation:
• It is the cheapest, easiest and most efficient tool
that the grower can use to change the greenhouse
climate.
• The study of natural ventilaton is quite complex
becaue it depends on the external climate conditions
and the geometry of the greenhouse and its vents,
however, after many years of study we know much
more on how to optimize it.
C02
RH/VPD
ET
TRENDS IN NATURAL VENTILATION
Ta
Tc
At night ventilation is also important to decrease humidity and to avoid
thermal inversion on clear nights
ONE OF THE MAIN PROBLEMS OF MEDITERRANEAN ARTISAN GREENHOUSES
INSUFFICIENT NATURAL VENTILATON
INSUFFICIENT VENTILATION
AREA AND HIGH GREENHOUSE
DENSITY
USE OF LOW POROSITY
INSECT PROOF SCREENS
INEFFICIENT VENTILATOR
DESIGNS
MOTOR FORCES OF VENTILATION
THERMALLY DRIVEN
VENTILATION 21
42
2
H
T
TgC
Sd
WIND DRIVEN VENTILATON
vCCS
wd2
DOMINATES IF V<2-3 m s-1
DOMINATES IF V>3 m s-1
Airflow characteristics under wind driven
ventilation
a. Windward ventilation b. Leeward ventilation
Montero et al.
Side wall ventilation
24.0 m4.0 m
3.6
m1
.2 m
0.9 m
100.0 m
24.0 m4.0 m
3.6
m1
.2 m
0.9 m
100.0 m
The effect of number of spans on greenhouse ventilation rate
(a) Fully open windward and leeward side vents and roof vents. (b) Only roof
Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.2 (2d, segregated, ske)
Dec 28, 2006
5.5
5.2
4.9
4.7
4.4
4.1
3.8
3.6
3.3
3
2.7
2.5
2.2
1.9
1.6
1.4
1.1
0.83
0.55
0.28
0.0048
Kacira et
al. (2004)
Puntos de medida
y = 0.096 + 0.20x (r = 0.85)
Ve
locid
ad
inte
rio
r a
ire
(m/s
)
Velocidad viento exterior (m/s) Velocidad viento exterior (m/s)
Filas Perpendiculares a paredes laterales (1.5 mH) Filas Paralelas a paredes laterales (1.5 mH)
y = 0.028 + 0.11x (r = 0.83)
Dirección del viento
Este Oeste
(Sase, 1989)
EFFECT OF INCREASING THE SLOPE OF THE SPANS
Wind velocity (m s-1) Tracer gas (4,4 m) CFD (4,4 m) CFD (4,9 m) CFD (5,4 m) CFD 5,9 (m)
2 7.7 7.7 7.8 7.9 7.8
3 10.2 9.7 11.9 14.1 15.3
4 12.7 11.6 19.6 21.5 21.6
5 15.1 14.2 23.3 27.4 29.5
6 17.6 17.4 25.8 32.4 35.1
Ventilation rate (m3 s-1)
Q = 5,62v
R2 = 0,96
Q = 5,28v
R2 = 0,97
Q = 4,46v
R2 = 0,96
Q = 2,95v
R2 = 0,93
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7
Velocidad del viento (m/s)
Cau
dal d
e v
en
tila
ció
n (
m3/s
)
Experimental (gas
trazador). (Pérez-
Parra et al., 2004)
CFD: 4.4 m
CFD: 5.4 m
CFD: 4.9 m
CFD: 5,9 m
Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.1 (2d, segregated, ske)
May 06, 2004
5.11e+00
4.86e+00
4.60e+00
4.35e+00
4.09e+00
3.84e+00
3.58e+00
3.33e+00
3.07e+00
2.81e+00
2.56e+00
2.30e+00
2.05e+00
1.79e+00
1.54e+00
1.28e+00
1.02e+00
7.68e-01
5.13e-01
2.57e-01
1.46e-03
Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.1 (2d, segregated, ske)
May 06, 2004
6.30e+00
5.99e+00
5.67e+00
5.36e+00
5.04e+00
4.73e+00
4.41e+00
4.10e+00
3.78e+00
3.47e+00
3.15e+00
2.84e+00
2.52e+00
2.21e+00
1.90e+00
1.58e+00
1.27e+00
9.51e-01
6.37e-01
3.22e-01
7.37e-03
Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.1 (2d, segregated, ske)
May 06, 2004
6.72e+00
6.38e+00
6.04e+00
5.71e+00
5.37e+00
5.04e+00
4.70e+00
4.37e+00
4.03e+00
3.69e+00
3.36e+00
3.02e+00
2.69e+00
2.35e+00
2.02e+00
1.68e+00
1.34e+00
1.01e+00
6.73e-01
3.37e-01
1.07e-03
Velocity Vectors Colored By Velocity Magnitude (m/s)FLUENT 6.1 (2d, segregated, ske)
May 06, 2004
6.91e+00
6.56e+00
6.21e+00
5.87e+00
5.52e+00
5.18e+00
4.83e+00
4.49e+00
4.14e+00
3.80e+00
3.45e+00
3.11e+00
2.76e+00
2.42e+00
2.07e+00
1.73e+00
1.38e+00
1.04e+00
6.91e-01
3.46e-01
7.68e-04
Standard slope 11,9º 19º
25º 30º
Contours of Static Temperature (k)FLUENT 6.1 (2d, segregated, ske)
Dec 02, 2004
3.12e+02
3.11e+02
3.10e+02
3.09e+02
3.08e+02
3.07e+02
3.06e+02
3.05e+02
3.04e+02
3.03e+02
Contours of Static Temperature (k)FLUENT 6.1 (2d, segregated, ske)
Dec 02, 2004
312
311
310
309
308
307
306
305
304
303
Contours of Static Temperature (k)FLUENT 6.1 (2d, segregated, ske)
Dec 02, 2004
3.12e+02
3.11e+02
3.10e+02
3.09e+02
3.08e+02
3.07e+02
3.06e+02
3.05e+02
3.04e+02
3.03e+02
Contours of Static Temperature (k)FLUENT 6.1 (2d, segregated, ske)
Dec 02, 2004
3.12e+02
3.11e+02
3.10e+02
3.09e+02
3.08e+02
3.07e+02
3.06e+02
3.05e+02
3.04e+02
3.03e+02
11,9º
25º 30º
Contours of custom-function-0FLUENT 6.1 (2d, segregated, ske)
Dec 07, 2004
9
8
7
6
5
4
3
2
1
0
Thermal gradient INT.-EXT. (ºC)
19º
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7
Velocidad del viento (m/s)
Tasa d
e v
en
tila
ció
n (
m3 s-1
)
Gas trazador
Modelo 2: 0,7 m
Modelo 1: 0,4 m
Modelo 3: 1 m
Modelo 4: 1,4 m
Modelo 5: 1,6 m
Modelo 6: 1,9 m
…increasing the size of the roof vetns has clear an important effect on the ventilation rate
At 4 m s-1, only vents with width higher to 1 m provide
acceptable air exchange values (>30 vol. h-1)
V=5 m/s; Alerón 0,73 m; Q = 14,22 m3/s ; Vel.( x=16 m) =0,234 m/s
Effects of size of roof ventilator on the ventilation rate-wind
speed relationship
V=5 m/s; Alerón 1,6 m; Q = 62,36 m3/s ; Vel.( x=16 m) =0,99 m/s
Effects of size of roof ventilator on the ventilation rate-wind
speed relationship
Results
Temperaturas (ºC) 22/07/2009
20
22
24
26
28
30
32
34
36
38
40
42
44
0:00:00 4:48:00 9:36:00 14:24:00 19:12:00 0:00:00 4:48:00
Hora del día
Te
mp
era
tura
(ºC
)
Temperatura exterior [ºC]
Temperatura interior nave 22 con blanqueo(ºC)Temperatura nuevo prototipo sin blanqueo [ºC]
Double roof vents per span
Most of the climate controllers keep both
vents opened and open and close all
leeward and all windward vents at the
same time. Is this the best management?
(Sase, 1983)
INTRODUCTION
To respond these questions we
need to measure temperature and
flow patterns generated in each
scenario…
High ventilation capacity with low winds
when greenhouse ventilates by thermal
effect (Baeza, 2009)
If wind velocity is v>2 m s-1
we know from previous works…
Natural ventilation systems appear to gain more attention in recent years due to
increased costs of energy and maintenance.
Natural ventilation is generally much cheaper than mechanical ventilation and
represents potential economical savings because less energy is needed for operations.
However, natural ventilation process and the control of ventilation rates is complex in
naturally ventilated greenhouses.
In addition, the natural ventilation itself may not be sufficient to provide desired
environment under certain conditions.
Thus, High Pressure Fogging (HPF) systems coupled with natural ventilation have
been studied in an aid to improve the performance on control of greenhouse
temperature and humidity (Arbel et al., 2006; Li et al., 2006; Li and Willits, 2008; Abdel-
Ghany and Kozai, 2006; Abdel-Ghany et al., 2006).
However, HPF with natural ventilation still presents some limitations of control.
One reason is lack of control of air flow and spray rates and advanced control
strategies for controlling ventilation and fogging events.
Also, the pressure of these kind of systems is usually constant, limiting control of
spray rates and pressure itself. Thus, here is a further need for research on developing
enhanced control strategies for natural ventilated greenhouses equipped with high
pressure and variable pressure fogging systems.