Agriculture Reference
In-Depth Information
years ago was of this type, the northern wall
providing thermal inertia. This type (passive
solar collection) has also been used in recent
decades as useful complementary equipment
for solar heating of buildings.
Greenhouses that are buried or semi-
buried in the ground allow for natural ther-
mal regulation with much lower oscillations
than in conventional greenhouses. Their
use has been limited to the cultivation of
ornamental species that need high tempera-
tures and humidity, or to greenhouses for
plant propagation. The areas where they are
built must not have a shallow water table or
be prone to flooding.
Depending on the cladding material we
may distinguish the following greenhouse
types:
(a)
(b)
(c)
(d)
Fig. 4.10. Some common types of gable-roof
greenhouses: symmetric multi-span (a),
asymmetric multi-span (b), simple single span (c),
and attached (lean-to) single span (d).
Glass
Although greenhouses are usually
located on flat terrain, in some cases (south
coast of Spain, for instance) they are built in
south-oriented slopes (in the northern hemi-
sphere), profiting from more favourable win-
ter radiation conditions than if they were
built on flat terrain. Although this location
limits the possibilities of mechanization, the
roof and sidewall surfaces exposed to the
north are lower, with the subsequent advan-
tage from the thermal point of view during
the winter. In a similar way, the soil surface
(or soil and terrace wall, if the greenhouse is
terraced) is higher with a positive incidence
on the greenhouse thermal inertia. In addi-
tion, the ratio of plastic cladding area:soil
area is reduced, limiting the thermal losses.
The disadvantages of this type of green-
house, besides the already mentioned diffi-
culty for mechanization, are the observed
temperature stratification, both during the
daytime (which requires the proper location
of ventilators) and during the night, due to
the higher weight of the cold air, which
stratifies on the lower parts.
Another type of greenhouse, common in
home gardens and as commercial green-
houses in China (solar greenhouse), is the
'lean-to' greenhouse where the greenhouse
leans against a wall. In the northern hemi-
sphere, the greenhouse is oriented towards
the south and the wall is on the northern side
(Fig. 7.5). The first greenhouse constructed
Plastic:
° Flexible film
° Rigid panel:
Simple
Alveolar
° Screens
Although today it is not a commonly
used form of classification, greenhouses can
be classified according to the minimum
temperature level that they can maintain,
so we can distinguish: (i) ' cold green-
houses ' (5-8°C); (ii) ' temperate greenhouses '
(12-15°C); and (iii) ' warm greenhouses '
(20-25°C).
4.6.3
Structure materials
The rigidity characteristics of the cladding
materials determine, to a great extent, the
shape of the greenhouse and the materials
from which it is made. The rigidity of glass
limits its use on curved-roof structures, so it
is normally used on straight-roof structures,
such as gable roofs. Panels, depending on the
type, only permit small curvatures, so their
use on curved-roof greenhouses is rare.
In a conventional greenhouse, a func-
tional structure that complies with the primary
purpose of holding the cladding material and
the predictable loads (snow, wind, trained
 
 
 
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