Agriculture Reference
In-Depth Information
8
Cooling
8.1
Introduction
Air cooling by conventional methods
(refrigerator circuit) is not economical,
under normal conditions.
At plant level, the first measure to limit
high temperatures is to irrigate properly, so
that the plants can transpire to the maximum
and decrease their temperature, complemen-
ted by efficient air renewal through ventilation.
The battle against high temperatures inside
the greenhouse is focused on decreas-
ing the energy inputs and eliminating their
excesses. If heating is used, the artificial
energy input (by the heating system) is
eliminated by turning it off. In low thermal
inertia systems (air heating) the response is
immediate, whereas in those with high
inertia (water heating) there is a delay in
the response.
The decrease in natural inputs is
focused, in practice, on limiting solar radi-
ation, by means of shading, inside or out-
side the greenhouse.
The increase in energy losses is
achieved with ventilation, natural (or static)
and forced (or mechanical), as a first step.
Every ventilation system can only decrease
the interior air temperature to the value of
the outside air if the renewed air has the
same humidity. This decrease of the interior
air temperature by ventilation is, in many
cases, enough to achieve acceptable thermal
levels inside the greenhouse when the exter-
nal air temperature is not excessive (i.e.
ventilation counteracts the greenhouse
effect). If the interior temperature must be
further decreased, active cooling methods
will have to be used; most commonly, by
evaporating water.
8.2
Function of Ventilation
Aeration, or ventilation, is the air exchange
between the greenhouse and the exterior.
This air exchange takes place through
the greenhouse openings (vents and slits)
(Photo 8.1). The air renewal allows the
evacuation of the excess heat and a decrease
in the air temperature, modifying the atmos-
pheric humidity (exchanging the interior air
with high water vapour due to plant tran-
spiration), and modifying the gas composi-
tion of the atmosphere (especially the CO
2
).
If the air leaving the greenhouse is dry,
the energy evacuated is very limited due to
the low specific heat of dry air (1 kJ kg
−1
°C
−1
,
at 20°C). If the outgoing air is humid, the
temperature decrease will be much higher,
as the energy evacuated with the humid
greenhouse air is much bigger (the energy
to evaporate 1 kg of water, or the latent
heat of water evaporation is 2445 kJ kg
−1
).
