Agriculture Reference
In-Depth Information
The distinction between a thermal and
a shading screen is not well determined,
because all screens act against all the radia-
tive losses and inputs, and growers prefer to
use a single polyvalent screen, so it is often
necessary to achieve a compromise in the
performance of the screen to make it useful
for both purposes (see Chapter 4).
8.9 Ventilation and Climate
Management
8.9.1 Temperature management
The efficiency of ventilation to decrease the
temperature depends on the amount of heat
to be removed (which in turn depends on
the solar radiation input), on the air exchange
rate and the state of the vegetation.
To achieve inside greenhouse tempera-
tures that are close to the external values,
air exchange rates of 20 volumes h −1 in
winter, 40 during the spring and 80 or more
during the summer are needed, provided
there is a crop that is transpiring normally
(Wacquant, 2000). The threshold tempera-
ture to begin operating natural ventilation
is usually set between 23 and 26°C, depend-
ing on the climate conditions and the
crops. In winter, the threshold temperature
to begin ventilation must be higher, 4-6°C
higher than the heating threshold tempera-
ture, with the aim of avoiding simultane-
ous heating and ventilation (ASAE, 2002),
although this might also be acceptable in
certain situations. When the external air is
very cold (<5°C) the air must be introduced
in such a way that, before reaching the
8.8
Other Cooling Methods
Soil and substrate cooling consists of circu-
lating cold water through a pipe or over a
carpet in contact with the soil or the sub-
strate. The water is cooled with a water/
water heat pump. It is an expensive system
and it is only used in very sophisticated
greenhouses.
During the summer months, circulating
a water film over the greenhouse cover cools
it and limits solar radiation, which is par-
tially absorbed, decreasing the greenhouse
temperature by up to 3°C (Breuer and Knies,
1995). The water use can be important in
warm climate areas. This technique, which
is usual in sophisticated greenhouses, has
not been used in Mediterranean greenhouses,
where it could be of interest (Photo 8.7).
Photo 8.7. The 'irrigation' of the cover to cool the greenhouse has not been used in the Mediterranean.
 
 
 
 
 
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