Agriculture Reference
In-Depth Information
plants, it must mix well with the internal
air (warmer) to avoid a sudden change in
the plant temperature.
In summer, under Mediterranean condi-
tions, when the maximum global external
radiation intensity varies between 900 and
1000 W m
−2
during the hours in the middle
of the day, the heat flux to be evacuated from
inside the greenhouse is close to 700 W m
−2
.
The crop uses 60-70% of the solar energy
for transpiration, leaving the remaining
210-280 W m
−2
to be removed (Wacquent,
2000), which involves very high air exchange
rates.
ventilation can be limited, tolerating a
small thermal increase.
The recommended management of
ventilation under Mediterranean condi-
tions (Sánchez-Guerrero
et al
., 1998)
establishes a set point of 25°C (or higher,
if there is CO
2
enrichment) and RH set
points of 75% (daytime) and 85% (at
night).
At the beginning of the crop cycle,
when there is less transpiration due to the
limited development of the plants, water
fogging is very advisable to decrease
temperatures.
8.10
Dehumidification
8.9.2
Humidity management
8.10.1
Associated heating
In the morning, when the Sun comes out,
the plants start transpiring, increasing the
water vapour content of the air inside the
greenhouse. The interior temperature can be
even lower than the ventilation set point
temperature. The greenhouse air is close to
humidity saturation and as the walls are
colder, water condenses on them first. Later,
condensation will occur in other parts of the
greenhouse and even on the coldest parts of
the plants such as the stems and fruits. A
small opening of the vents will evacuate a
large amount of the air saturated with
humidity, decreasing this condensation.
Sometimes in heated greenhouses, for
energy-saving reasons, low temperature set
points are used, which may notably increase
the water condensation before dawn. To
alleviate this situation it is recommended
that the set points are increased just before
dawn.
The efficiency of ventilation to
decrease the air humidity depends on the
state of the entering air. In winter and
beginning of the spring, when the external
air is cold, and with low humidity, a mod-
erate air exchange rate is sufficient to dehu-
midify, especially if it is associated with a
heat supply.
In summer, when the external air is
hot and dry, high ventilation may cause a
large drop in humidity inside the green-
house.
When there is no specific dehumidifica-
tion equipment (such as in the case of most
greenhouses), dehumidification is achieved
by heating and ventilating. This method is
efficient, but it requires very high energy
consumption and a very powerful heating
system. The energy consumption for dehu-
midification through this procedure may
involve 15% of the total energy used for
heating well-insulated greenhouses in the
south of France (Baille, 1999). When the
air is heated, the RH decreases as the satu-
ration vapour pressure increases. Later
ventilation avoids a temperature rise and
excess water vapour accumulated is evacu-
ated, introducing fresh air of low water
content.
8.10.2
Dehumidification systems
Conventional systems, that use heat pumps
or a refrigerator circuit, are not economi-
cally feasible (Urban, 1997a). Another
option is to circulate the humid air through
a hygroscopic fluid (calcium chloride, tri-
ethylene glycol) that absorbs part of the
water vapour in the air. Periodically, the
fluid must be heated to regenerate it. Its
profitability is not clear.
To
avoid
an
excessive
drop,
