DESIGN OF GREENHOUSE NATURAL VENTILATION SYSTEMS(I) E. Baeza, J.C. Lopez, J.I. Montero EUPHOROS PROJECT WORKSHOP SICILY (RAGUSA) OCTOBER 6 2011

INTRODUCTION

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

Mediterranean greenhouses use plastic films as cladding and investment is moderate, climate control limited to natural ventilation and shading (whitening)

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.

MAIN GREENHOUSE TYPES IN THE MEDITERRANEAN BASIN

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

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.

TRENDS IN NATURAL VENTILATION

C02

ET

Ta Tc RH/VPD

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 

S  T H   Cd  2g  2 T 4  

12

DOMINATES IF V3 m s-1

Airflow characteristics under wind driven ventilation

a. Windward ventilation

b. Leeward ventilation

Montero et al.

Side wall ventilation 3.6 m 1.2 m

0.9 m

0.0 10

Kacira et al. (2004)

m

4.0 m

24.0 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 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

Velocity Vectors Colored By Velocity Magnitude (m/s)

Dec 28, 2006 FLUENT 6.2 (2d, segregated, ske)

Suggestions to improve natural ventilation.

Sase (2006)

Puntos de medida

Dirección del viento

Este

Oeste

Velocidad interior aire (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)

y = 0.096 + 0.20x (r = 0.85)

Velocidad viento exterior (m/s)

Velocidad viento exterior (m/s)

(Sase, 1989)

EFFECT OF INCREASING THE SLOPE OF THE SPANS

Wind velocity (m s-1) 2 3 4 5 6

Ventilation rate (m 3 s-1) CFD (4,4 m) CFD (4,9 m) CFD (5,4 m) 7.7 7.8 7.9 9.7 11.9 14.1 11.6 19.6 21.5 14.2 23.3 27.4 17.4 25.8 32.4

Tracer gas (4,4 m) 7.7 10.2 12.7 15.1 17.6

40 35 Caudal de ventilación (m 3/s)

Experimental (gas trazador). (PérezParra et al., 2004)

Q = 5,62v R2 = 0,96

30

CFD: 4.4 m

Q = 5,28v 25

R = 0,97

20

Q = 4,46v

15

R = 0,96

2

CFD: 5.4 m

2

CFD: 4.9 m 10

Q = 2,95v R2 = 0,93

5

CFD: 5,9 m

0 0

1

2

3

4

5

Velocidad del viento (m /s)

6

7

CFD 5,9 (m) 7.8 15.3 21.6 29.5 35.1

5.11e+00

6.30e+00

4.86e+00

5.99e+00

4.60e+00

5.67e+00

4.35e+00

5.36e+00

4.09e+00

5.04e+00

3.84e+00

4.73e+00

3.58e+00

4.41e+00

3.33e+00

4.10e+00

3.07e+00

3.78e+00

2.81e+00 2.56e+00

3.47e+00 3.15e+00

2.30e+00

2.84e+00

2.05e+00

2.52e+00

1.79e+00

2.21e+00

1.54e+00

1.90e+00

1.28e+00

1.58e+00

1.02e+00

1.27e+00

7.68e-01

9.51e-01

5.13e-01

6.37e-01

2.57e-01

3.22e-01

1.46e-03

7.37e-03

Standard slope 11,9º

Velocity Vectors Colored By Velocity Magnitude (m/s)

May 06, 2004 FLUENT 6.1 (2d, segregated, ske)

19º

Velocity Vectors Colored By Velocity Magnitude (m/s)

6.72e+00

6.91e+00

6.38e+00

6.56e+00

6.04e+00

6.21e+00

5.71e+00

5.87e+00

5.37e+00

5.52e+00

5.04e+00

5.18e+00

4.70e+00

4.83e+00

4.37e+00

4.49e+00

4.03e+00

4.14e+00

3.69e+00 3.36e+00

3.80e+00 3.45e+00

3.02e+00

3.11e+00

2.69e+00

2.76e+00

2.35e+00

2.42e+00

2.02e+00

2.07e+00

1.68e+00

1.73e+00

May 06, 2004 FLUENT 6.1 (2d, segregated, ske)

1.38e+00

1.34e+00

1.04e+00

1.01e+00

6.91e-01

6.73e-01

3.46e-01

3.37e-01

7.68e-04

1.07e-03

25º Velocity Vectors Colored By Velocity Magnitude (m/s)

30º

May 06, 2004 FLUENT 6.1 (2d, segregated, ske)

Velocity Vectors Colored By Velocity Magnitude (m/s)

May 06, 2004 FLUENT 6.1 (2d, segregated, ske)

Thermal gradient INT.-EXT. (ºC) 312

9

311

3.12e+02

310

8 3.11e+02

309 3.10e+02

7

308 3.09e+02

6

307

3.08e+02 3.07e+02

11,9º

306

19º

5

3.12e+02

3.12e+02 305

3.06e+02

3.11e+02

3.05e+02 4

3.11e+02 304

3.10e+02 3.04e+02

3.10e+02 303

3 3.09e+02 3.03e+02

3.09e+02 Contours of Static Temperature (k)

3.08e+022

Contours of Static Temperature (k)

3.07e+02

1

Dec 02, 2004 FLUENT 6.1 (2d, segregated, ske)

Dec 02, 2004 3.08e+02 FLUENT 6.1 (2d, segregated, ske)

25º

3.07e+02

30º

3.06e+02

3.06e+02 3.05e+020

3.05e+02 3.04e+02

3.04e+02

Contours of custom-function-0

Dec 07, 2004

…increasing the size of the roof vetns has clear an important effect on the ventilation rate 100

Tasa de ventilación (m 3 s-1)

90 80 70

Gas trazador

60

Modelo 2: 0,7 m Modelo 1: 0,4 m

50

Modelo 3: 1 m

40

Modelo 4: 1,4 m

30

Modelo 5: 1,6 m Modelo 6: 1,9 m

20 10 0 0

1

2

3

4

5

6

7

Velocidad del viento (m/s)

At 4 m s-1, only vents with width higher to 1 m provide acceptable air exchange values (>30 vol. h-1)

Effects of size of roof ventilator on the ventilation rate-wind speed relationship

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

New greenhouse designs with improved ventilation

Results

Temperatura (ºC)

Temperaturas (ºC) 22/07/2009

44 42 40 38 36 34 32 30 28 26 24 22 20 0:00:00

4:48:00

9:36:00

14:24:00

19:12:00

Hora del día Temperatura exterior [ºC] Temperatura interior nave 22 con blanqueo(ºC) Temperatura nuevo prototipo sin blanqueo [ºC]

0:00:00

4:48:00

INTRODUCTION

Let s have a look to an example to illustrate…

INTRODUCTION Double roof vents per span 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… (Sase, 1983)

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? To respond these questions we need to measure temperature and flow patterns generated in each scenario…

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; AbdelGhany 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.