Agriculture Reference
In-Depth Information
availability limit the options when building
a greenhouse (Briassoulis et al ., 1997a), so
there is a certain trend among growers to
build traditional greenhouses (Von Elsner
et al ., 2000b).
Relative to plastic-film greenhouses,
the most important aspects to achieve are
those detailed in the following paragraphs
(Zabeltitz, 1990, 1999; Von Elsner et al .,
2000a, b).
Besides the proper structural resistance
to the wind, but also to other predictable
loads (snow, crops which are trained to
hang, auxiliary equipment), the greenhouse
must be built in such a way that the plastic
film will remain well fastened, airtight, and
without wrinkles, to avoid breaks caused by
the wind. It must, as well, be easy to change
the film. For this, the fastening system must
be simple and efficient. The increasing costs
of mounting the film and the plastic materi-
als have favoured the use of special films
with several years' durability. For longer
durability, if possible, the structural ele-
ments susceptible to heating up by solar
radiation which are in contact with the
plastic film must be insulated, because
excess temperatures contribute to shorten-
ing the shelf life of the plastic film.
When arcs or metal frames are used, the
separation between them will depend on
the predictable loads (wind, snow), nor-
mally does not exceed 3 m.
The greenhouse must be airtight, to pre-
vent night cooling in those climates in
which low night temperatures are expected,
as well as to prevent undesirable leakage of
CO 2 . A proper ventilation system is needed,
with airtight vents. The entrance of water
from rainfall must be avoided.
Its volume must be large enough, not
only to obtain a higher thermal inertia, but
also to allow for crops that are trained to
grow up high supports, and proper move-
ment of the inside air necessary for natural
ventilation. The unitary volume of the
greenhouse is the quotient between the
greenhouse inner volume (m 3 ) and the area
that it covers (m 2 ), being equivalent to the
average height.
Collection of rainfall water by means
of gutters, for its later storage and use for
irrigation, is not only of interest in areas of
low rainfall, but also because the excellent
quality of rain water makes it especially
valuable for soiless cultivation, a technique
for fast growth. The gutter must be 4 cm
larger than the diameter of the drainpipe
and must have a slope of 1% to avoid over-
flows (the minimum slope must be higher
than 0.2% in any case). The drainpipes
must have a cross-section of 7 cm 2 for each
10 m 2 of cover area that is to be drained,
which caters for rainfall intensities of up to
75 mm h −1 (Aldrich and Bartok, 1994).
To avoid water dripping over the crops
from condensation on the inner surface of the
cover, it is important to build the greenhouse
with roof angles greater than 26° (such angles
also allow snow to run off the cover), and
have an appropriate collection system, or to
use anti-dripping plastic film. In unheated
greenhouses, where climate control is quite
limited, the slope of the roof becomes rele-
vant to avoiding condensed water dripping
from the roof cover; roofs with ogive shape
might be of interest (Fig. 4.9).
Likewise, as a general rule, the green-
house must maximize solar radiation trans-
mission, at least in winter (when it is lower),
for which proper roof geometry and orienta-
tion are fundamental.
4.9.3 Design criteria in areas
with a Mediterranean climate
The most limiting climate conditions for
greenhouse cultivation in Mediterranean
climates are: (i) low night temperatures
in winter; (ii) high daytime temperatures;
(iii) high ambient humidity at night and
low values during the day; and (iv) CO 2
depletion during the day (Zabeltitz, 1999;
Von Elsner et al ., 2000a, b).
Therefore, it is especially necessary to
achieve efficient ventilation, which allows
for alleviation of the thermal excesses and
extreme humidities, and prevents CO 2 defi-
cits. Depending on the type of greenhouse
and climate conditions it is advisable that
the ventilation area is up to 30% of the ground
area of the greenhouse. The increasing use of
 
 
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