2012/10/31

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1

Greenhouse Ventilation Greenhouse Ventilation

and Managementand Management

Controlled Environment Agriculture

Sadanori Sase

Agricultural Environment Engineering Division

National Institute for Rural Engineering, NARO

2

Introduction

Cooling greenhouse air is more important

Introduction

Cooling greenhouse air is more important

� Greenhouse production is expanding in the regions

under mild climate.

� Year-round production is one of the primary concerns

to increase efficiency and productivity.

� The production of plants that require lower growing

temperatures is increasing.

� Progress in the greenhouse design and control

technologies

� Increase in eaves (gutter) height

� Wider vent openings

� Open-roof design

� Fog cooling in combination with natural ventilation

3

Ventilation

Roles and advantages

Ventilation

Roles and advantages

� Roles

� Prevent excessive temperature rise under mild climate

� Supply CO2 from external air

� Humidity control

� Airflow affects the plant growth (gas and energy

exchange between plants and surrounding air)

� Proper ventilation necessary for evaporative cooling

and/or shading systems

� Advantages of natural ventilation

� High ventilation rate, but depend on structures and

outside weather conditions

� Uniformity of environment

� Less electric energy and quiet

4

Air movement caused by ventilation affects the uniformity of greenhouse

environment (particularly, air temperature) and plant growth/quality

Air movement caused by ventilation affects the uniformity of greenhouse

environment (particularly, air temperature) and plant growth/quality

VentilationVentilation

Sensible heat

Solar radiation

CO2

Outside wind

Plants

Latent heat (water vapor)

Air movementAir movement

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Previous studies of air movement based on

advancements in velocity vector measurement

Previous studies of air movement based on

advancements in velocity vector measurement

� Field experiments

� Sonic anemometer systems

� Laboratory tests

� Wind tunnel tests

� Particle imagery velocimetry (PIV)

� Rapid progress in computational fluid dynamics (CFD)

� Advantages include investigation at many points of interest, easy

change in weather and structural conditions, visualization of

airflow, and saving of time, labor and cost.

� The accuracy has been improved by verification tests and

improvement of related models.

� More recently, focus on the internal airflows with plants and the

interactions between the plant canopy and the ventilated air by

incorporating the heat and mass balance models.

Camera

Laser

Wind tunnel

6

Topics and questions in relation to natural ventilationTopics and questions in relation to natural ventilation

� Sufficient vent openings and their locations

� Efficient structures and covering materials including

open-roof greenhouses

� Eaves (gutter) height increase ventilation performance?

� Side ventilators are effective?

� Horizontal airflow can be expected. But, when the

greenhouse width is increased, the effect may be

reduced.

� Effect of inside airflow on uniformity and plant growth

� Effect of tall plants

� Reduction in ventilation by insect screens

� Method and strategy of ventilation control

� Thermal comfort of workers

7

Basis for preventing excessive rise in greenhouse temperatureBasis for preventing excessive rise in greenhouse temperature

� Ventilation to exchange internal air with cooler external air

� Shading to decrease incoming solar radiation

� Both not achieve lower internal air than external air.

� Cooling to cool internal air below external air

� Evaporative cooling technique is the most practical and inexpensive in operating cost.

� Shading and/or evaporative cooling systems function well in combination with ventilation.

� The temperature rise does not linearly decrease as the ventilation rate increases, but is nearly proportional to the sensible heat converted from incoming solar radiation.

� Shading reduces the photosynthetically active radiation, and restrict airflow and natural ventilation when the shading curtains are extended in a greenhouse.

Ventilation

Shading

Sensible heat

Latent heat

8

Principle of ventilation based on Bernoulli equationPrinciple of ventilation based on Bernoulli equation

G = α A ( 2 g ρ ∆P )1/2

G airflow rate (kg/s)α discharge coefficientA area of openingg gravity acceleration (m/s2)ρ density of air (kg/m3)∆P pressure difference (kg/m2)

= Pi- P

w+ P

b

Pi

pressure on floor surfaceP

wwind pressure = C

w(ρ/2g)V2

Cw

wind pressure coefficientV wind velocity

Pb

buoyancy ∝ H, ∆TH height of opening∆T temperature difference

between inside and outside

Buoyancy (chimney) effectWind effect

G

H

∆P

� Natural ventilation rate varies linearly with external

wind velocity and area of vent openings, while it also

varies linearly with the square roots of height of

openings and temperature rise.

� For the design purpose, wider openings are effective

in increasing the ventilation rate, particularly for the

conditions that wind velocity is low.

� Recommended total area of openings of the floor

area of a greenhouse in general.

� 15-20% for side vents and 15-20% for roof vents

(ASABE, 2003)

� 33% for a large-sized greenhouse with only roof

vents (The Electricity Council, 1975)

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Effects of greenhouse height and vent opening area on temperature

rise in multi-span greenhouses under natural ventilation

Effects of greenhouse height and vent opening area on temperature

rise in multi-span greenhouses under natural ventilation

� The eaves height of large-sized multi-span greenhouses such as Venlo

greenhouses has been increased.

� The taller height resulting in greater greenhouse capacity may prevent

the quick change in environment and improve the spatial uniformity.

Other advantages are to promote the ventilation caused by chimney

effect and the mixing of cooler incoming air through roof vents with

internal air, because the location of roof vent openings is raised.

� When the side walls with high eaves are widely opened, external wind

is expected to promote horizontal airflow through the greenhouse

space. However, the tall plants such as tomato plants seem to restrict

the external air flowing in. Furthermore, the area of side openings per

greenhouse floor area decreases as the width of a greenhouse

increases.

� Therefore, the roof vents are more important for large-sized

greenhouses, especially the increase in the area of roof vent openings.

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従来の丸屋根型温室とMX-II（Van Wingerden社、米国）

Introduction of open-roof greenhouses allows the roofs to be entirely openedIntroduction of open-roof greenhouses allows the roofs to be entirely opened

Main advantages of openMain advantages of open--roof greenhousesroof greenhouses

� During warm(er) conditions, the

greenhouse temperature closely tracks

outside temperatures with little or no

energy requirements (to operate the fans).

� Spring plants can be easily hardened off

by opening the roof.

The crop is grown on movable benches and rolled out of the greenhouse to receive maximum light and cooler conditions.

Traditional fan-ventilated greenhouse

The MX-II open-roof greenhouse with roof panels opening at the peaks

Rolled out

(Mears, 2003)

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18161412108622

24

26

28

30

32

34

36

38

40

July 2, 1999

Fan ventilated

greenhouse

Comparison of air temperature between a open-roof greenhouse (MX-II) and a fan ventilated greenhouseComparison of air temperature between a open-roof greenhouse (MX-II) and a fan ventilated greenhouse

Time (h)

Tem

pera

ture

(°C

)

MX-II at a

height of 2.4 m

MX-II at a height

of 1.2 m

Outside temperature

Open-roof greenhouse (MX-II)

Fan ventilated greenhouse

with a ventilation rate of one

volume change per minute

(Roberts et al., 1999)

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Open-roof greenhouse designs from across the world

Open-roof greenhouse designs from across the world

Roofs hinged at one gutter and the ridge

Roofs hinged at the gutters

Roll-up roof coverings

Roof halves hinged at the ridge

Retractable roof coverings

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Sunlight distribution in an open-roof greenhouse

when roofs are fully opened

Sunlight distribution in an open-roof greenhouse

when roofs are fully opened

反射透過 ReflectedTransmitted

Shadow by roof Direct Direct +

reflectedShadow by gutter

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Shading curtain increases inside temperature

and WBGT in an open-roof greenhouse

Shading curtain increases inside temperature

and WBGT in an open-roof greenhouse

181614121086

15

20

25

30

35

40

Te

mp

era

ture

(˚C

)

Time (h)

181614121086

15

20

25

30

35

40

WB

GT

(˚C

)

Time (h)

Outside

100% open

50% open

Shading curtain position

0% open

50％ open 100% open

Shading curtain position

0% open

(Ishii et al., 2001)

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Simple model combining buoyancy/wind effect

and heat balance

Simple model combining buoyancy/wind effect

and heat balance

� Assumption

� Only roof ventilators are equipped

� Inflow area is equal to outflow area

� Wind/buoyancy effect numerical model

+

� Sensible heat balance model

� Then the ventilation rate and the temperature

difference between inside and outside can be

calculated simultaneously.

(Boulard and Baille, 1995; Kittas and Boulard, 1997)

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1.00.80.60.40.20.0

0

2

4

6

8

10

12

Area ratio of roof ventilator opening to floor

Internal net radiation 600 W/m2

Conversion ratio into sensible heat 0.5

Height of roof ventilator

opening 4 m

Wind velocity 1 m/s

Tem

pera

ture

ris

e (

˚C)

(Sase and Okushima, 1998)

Effect of roof vent opening area on temperature rise in multi-span

greenhouses equipped with only roof vents under natural ventilation

Effect of roof vent opening area on temperature rise in multi-span

greenhouses equipped with only roof vents under natural ventilation

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(Sase and Okushima, 1998)

5.04.54.03.53.02.52.0

3

4

5

6

7

Height of roof ventilator opening (m)

Wind velocity

1 m/s

2 m/s

Internal net radiation 600 W/m2

Conversion ratio into sensible heat 0.5

Area ratio of roof ventilator opening to floor 0.2

0 m/s

Tem

pera

ture

ris

e (

˚C)

Effect of height of roof vent opening on temperature riseAn increase in height is effective when low wind velocity.

Effect of height of roof vent opening on temperature riseAn increase in height is effective when low wind velocity.

18

4 m

3.6

m

100 m

Wind direction

Can the side vents contribute to an increase in ventilation?Can the side vents contribute to an increase in ventilation?

� Ratio of roof vent opening area to floor

area: 9.6%

� The reduction is reasonably explained by

the fact that the area of side vent

openings is kept constant and the area

per greenhouse floor decreases with an

increase in the span number.

30241812600.0

1.0

2.0

3.0

30241812600.00

0.05

0.10

0.15

0.20

Fully open side vents

Number of spans Number of spans

Without side vents

Ve

ntila

tio

n ra

te (

AE

min

-1)

(Kacira et al., 2004)

2 m s-1

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Airflow in multi-span greenhouses with only roof vents

Wind effect in a Venlo greenhouse

Airflow in multi-span greenhouses with only roof vents

Wind effect in a Venlo greenhouse

1.0 m/s

� An air circulation with reverse flow above the floor was induced.

� The windward ventilator opening on the windward span showed the most significant effect on the intensity of circulation.

� On the other hand, the circulation was much weaker when the windward ventilators were closed.

Flow vector image obtained by PIV

(Okushima et al., 1998)

Airflow pattern drawn from observation

Staggered arrangement

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PIV computed vectors of airflow distribution in 6-span naturally

ventilated Venlo and open-roof greenhouses

PIV computed vectors of airflow distribution in 6-span naturally

ventilated Venlo and open-roof greenhouses

0.5m/s

0.5m/s

3.5m/s

Hei

gh

t (m

)1.21.00.80.60.40.20.0

0.2

0.1

0.0

1.21.00.80.60.40.20.0

0.2

0.1

0.0

Width (m)H

eight

(m)

( Lee et al., 2003)

VenloVenlo

OpenOpen--roofroof

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(Lee et al., 2000)

Airflow Animation Computed by CFD for a Venlo Greenhouse

� The entering air induces two weak eddies.

� The inside airflow is relatively turbulent.

� A portion of the entering air through the windward ridge

ventilator openings does not reach the crop canopy.

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1818--spanspan

2424--spanspan

Reversed flow

Reversed flow

(Kacira et al., 2004)

4 m

3.6

m

100 m

Wind direction

Effect of increased width (more spans) on airflows in a gothic greenhouseEffect of increased width (more spans) on airflows in a gothic greenhouse

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The temperature distribution has a close relationship

with internal airflow caused by natural ventilation

The temperature distribution has a close relationship

with internal airflow caused by natural ventilation

� Low temperature occurs in the upstream region

where ambient air enters.

� High temperature occurs in the downstream region of

airflow.

� Higher temperature occurs in the secondary

circulation) with low-velocity.

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Temperature distribution caused by air circulation in a 6-span Venlo greenhouse

Non-dimensional air temperature difference, (T-To)/(T-Ts) with 1 m high canopy

Temperature distribution caused by air circulation in a 6-span Venlo greenhouse

Non-dimensional air temperature difference, (T-To)/(T-Ts) with 1 m high canopy

0.15 0.20 0.400.25 0.30 0.35

4

1

2

3

0

0.0 6.4 12.8 19.2X (m)

4

1

2

3

0

0.0 6.4 12.8 19.2X (m)

4

1

2

3

0

0.0 6.4 12.8 19.2X (m)

4

1

2

3

0

0.0 6.4 12.8 19.2X (m)

V = 0 m/s V = 1.9 m/s

V = 3.8 m/s

Z (

m)

Z (

m)

Z (

m)

Z (

m)

(Okushima et al., 2000)

The highest air temperatures occurred in the windward space at

wind velocity of 1 m s-1. The air circulation above the crop canopy

became weaker as the crop height was increased.

V = 1.0 m/s

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Airflow as affected by plantsAirflow as affected by plants

� When the tall plants such as tomato plants are grown

in a greenhouse, the plant arrangement including

plant density and canopy structure affects the internal

airflow and the consequent ventilation performance.

� However, most of wind tunnel tests and CFD studies

to investigate the internal airflow have been carried

out for empty greenhouses.

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Effect of row direction of tomato plants on air velocity at a wind direction of EEffect of row direction of tomato plants on air velocity at a wind direction of E

Measurement Location

3210

0.0

0.2

0.4

0.6

0.8

3210

0.0

0.2

0.4

0.6

0.8

y = 0.096 + 0.20x (r = 0.85)

Inte

rna

l Air

Ve

locity (

m/s

)

External Wind Velocity (m/s) External Wind Velocity (m/s)

Rows Perpendicular to Side Walls (1.5 mH) Rows Parallel to Side Walls (1.5 mH)

y = 0.028 + 0.11x (r = 0.83)

Wind Direction

East West

(Sase, 1989)

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Airflow difference between without and with plant canopy

(Kacira et al., 2004)

WITHOUT

Plant Existence

WITH

Plant Existence

Plant row

The magnitudes of air velocities

were reduced dramatically due to

the drag effect of the plants, and

the air tended to move upward,

toward the roof opening in the

leeward span of the greenhouse.

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Determination of porosity parameters for tomato canopy by wind tunnel testingDetermination of porosity parameters for tomato canopy by wind tunnel testing

-1 0 1 2 3 4 5

1

2

0.90.8

1.01.1 1.2

1.3 1.4

1.2

1.5

0.6

0.5

0.4

Distance (m)

He

ight

(m)

∂P/∂x = ρ L CD u2

P pressure loss

ρ air density

L leaf area density

CD drag coefficient (= 0.31 for

the tomato canopy)

u air velocity

(Sase et al., 2012)

15 m/s

4 m

3 m

20 m

Test section

スパイヤー

ラフネスブロック

Spire

Roughness block

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Effect of Vent Configuration on Ventilation Rates for the Greenhouse

and the Plant Canopy Zone in a Two-span Greenhouse

Effect of Vent Configuration on Ventilation Rates for the Greenhouse

and the Plant Canopy Zone in a Two-span Greenhouse

To exclude the ventilated air that did not reach the

plant canopy, the ventilation rate for plant canopy

zone was defined.

12 3

4Wind

5

Wind direction

Ventila

tion r

ate

(m

3m

-2m

in-1

)

8

6

4

2

0

Greenhouse

Plant canopy zone

(Kacira et al., 2004)

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Ventilation as affected by insect screensVentilation as affected by insect screens

� Insect screens with fine mesh have been applied toexclude the insect vectors that cause virus diseases.

� The tobacco whitefly, Bemisia tabaci, which attacksa wide range of ornamental and vegetable crops, hasbeen one of the most serious problems.

� Since the insect screens restrict the airflow byincreasing the airflow resistance, air temperature risecaused by reduction in ventilation and less airflow inthe screened greenhouses are the major concern ofgrowers, particularly in the naturally-ventilatedgreenhouses under mild climate.

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Effect of screen discharge coefficient on natural ventilation rate and temperature rise

no screen0.80.60.40.20.00

10

20

30

40

50

60

70

80

3

4

5

10

15

Screen discharge coefficient

Ventila

tion r

ate

(/h

)

Tem

pera

ture

ris

e (°C

)

Internal net radiation 500 W/m2

Conversion ratio into

sensible heat 0.5

Wind velocity 1.5 m/s

Angle of opening 30 °

drawn from Sase and Christianson (1990)

3.9

2.1

7.2 Dimensions in m

Screen

(Floriade, 2002)

(Burlington, 1996)

For example, the discharge coefficient is 0.34 for a 60 mesh 0.15 mm stainless

steel wire screen that has 60 threads/inch (24 threads/cm) in each direction.

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Methods to improve the ventilation efficiency of

screened greenhouses by reducing the combined

resistance of vent openings and screens to airflow

Methods to improve the ventilation efficiency of

screened greenhouses by reducing the combined

resistance of vent openings and screens to airflow

� Since the screen airflow resistance is related to the

porosity of screen, the use of thinner threads with a

constant hole size is a method to increase the

porosity.

� An increase in the vent openings where screens are

placed is an effective alternative method.

� If the vents are fixed and have no possibility to be

reconstructed wider on the occasion of installation of

screens, an increase in the area of screen itself is a

practical alternative to reduce the airflow resistance

and increase ventilation rate.

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Effect of Reynolds number on pressure loss coefficient of

insect screens with a nominal pore size of 0.4 mm

Effect of Reynolds number on pressure loss coefficient of

insect screens with a nominal pore size of 0.4 mm

200150100500

0

2

4

6

8

10

Re

At a wind velocity of 3 m/s, Re is 40-100 for the pore size of 0.2-0.5 mm.

Thread diameter 0.23 mm

Pore size 0.39 mm

Porosity 0.40

(Tamaki et al., 2009)

∆P = Fs 1/2 ρ V2

∆P pressure loss (Pa)

Fs pressure loss coefficient (-)

ρ air density (kg/m3)

V velocity (m/s)

Thread diameter 0.18 mm

Pore size 0.41 mm

Porosity 0.49

Pre

ssure

loss c

oeffic

ient, F

s

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An increase in the area of screen itself to

reduce the airflow resistance and increase

ventilation

An increase in the area of screen itself to

reduce the airflow resistance and increase

ventilation

Horizontal installation at an

eaves height (Sase et al., 2008)

Pre-formed concertina-shape

(Bailey, 2003)

強制換気で内側にパッド

(Both, 2004)

Installation outside the sidewall

with ventilation inlet (Both, 2004)

V-shape screen installation for the air inlets

(Mears and Both, 2000)

Typical screening

installation

vent window

screen

poly lock

extrusion

pipe for weight

pipe for

weight

35

Thank you very much

for your attention

Thank you very much

for your attention