Agriculture Reference
In-Depth Information
outside the greenhouse generate these pres-
sure gradients. The air densities are affected
by the temperature and, to a lesser extent, by
the air composition (the humidity especially).
As ventilation affects the conditions of
the confined air, essential in the 'green-
house effect', the knowledge of the ventila-
tion flux is fundamental in the management
of the greenhouse climate (Bot and Van de
Braak, 1995).
The efficiency of ventilation, quanti-
fied by the air exchange rate (
R
), depends on
the climate conditions: (i) external wind
velocity and direction; and (ii) temperature
difference between inside and outside the
greenhouse. These two effects, the wind
effect and the thermal (buoyancy) effect,
generate pressure differences which force
the air to move (natural convection), from a
high pressure area to a low pressure area.
The ventilation efficiency also depends on
the characteristics of the openings (area and
position) and of the canopy (arrangement of
the crop rows in relation to the sidewall
vents).
8.3
How Airtight is the Greenhouse?
A greenhouse is not an airtight construc-
tion, suffering minor or major leakage losses.
The importance of the exchange due to leak-
ages depends on the quality of the construc-
tion and varies a lot with the wind velocity
(see Table 7.1). When the wind is weak, the
difference between the internal and the
external temperature is the main influence.
The losses increase with wind velocity.
How airtight a greenhouse is may be
measured by calculating the air renewal
coefficient with the help of a tracer gas or by
creating a pressure differential between the
inside of the greenhouse and the outside
(i.e. lower or higher pressure inside the
greenhouse compared with outside). It can
also be quantified by means of the thermal
balance.
The advantages of an airtight green-
house are: (i) a decrease in the thermal losses
(and therefore an increase in the amount of
energy saved); and (ii) a decrease in the CO
2
leakages, if carbon enrichment is practised.
The disadvantages of a greenhouse being
too airtight are the build-up of air humid-
ity and the higher risk of toxicity in the
case of pollution or pesticide application.
The existence of temperature gradients pow-
ers the convective movements, as the warm
air rises and the cold air descends. When
there is no wind, the air exchange rate depends
on temperature difference alone between the
interior and the exterior (Fig. 8.2).
A roof opening favours ventilation
(chimney effect) (Fig. 8.2). The efficiency of
the roof ventilation depends on the green-
house height. Due to the chimney effect the
taller greenhouses ventilate better, so it is
advisable to build them at least 3 m high
(Urban, 1997a).
The effect of the temperature gradient
on ventilation is important with weak winds,
high radiation and limited openings.
In Mediterranean greenhouses, the ther-
mal (buoyancy) effect is of little importance
in ventilation if the wind velocity exceeds
1-2 m s
−1
(Muñoz, 1998; Pérez-Parra, 2002),
8.4
Natural Ventilation
Natural ventilation allows for the renewal
of the interior hot air by external fresh air. It
is achieved by means of permanent or tem-
porary openings in the roof, in the sidewalls
or in the front walls. It is the cheapest and
most commonly used system.
Fig. 8.2.
Greenhouse ventilation fluxes when the
wind velocity is zero and there is only a thermal
effect.
