DESIGN OF GREENHOUSE NATURAL VENTILATION SYSTEMS · PDF fileDESIGN OF GREENHOUSE NATURAL...

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.



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.


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

FRUITS WITH CRACKING

BLOSSOM END ROT

BOTRYTIS, BACTERIA AND

OTHER DISEASES

ASSOCIATED TO HUMIDITY

EXCESSES.

VENTILATING IS ALSO IMPORTANT…

AND THERMAL CONFORT OF THE

WORKERS

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

Maximizing the screened area

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)

Suggestions to improve natural ventilation.

Sase (2006)

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


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


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


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


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


Dec 02, 2004

312

311

310

309

308

307

306

305

304

303


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


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

New greenhouse designs with improved

ventilation

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]

Let s have a look to an example to illustrate…

INTRODUCTION

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.

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