Agriculture Reference
In-Depth Information
The availability of labour will deter-
mine the level of mechanization and auto-
mation. Whether the crop is sedentary
(species whose cycle is set in the same
place) or mobile (pot plants) is another
aspect to consider, especially with regard to
optimizing the use of the space.
(a)
(b)
4.6.2
Greenhouse types
There are different classifications of the
greenhouse types. According to their archi-
tectural shape we can distinguish two basic
types: (i) a single-module or monospan
greenhouse, consisting of a greenhouse iso-
lated from other structures; and (ii) a multi-
module or multi-span greenhouse, formed
by a series of gutter-connected greenhouses.
The multi-span types allow for better use of
the soil, are cheaper to construct and to
heat, and having a lower ratio between the
cladding surface and the soil surface.
With regards to roof geometry there are
many variants (Fig. 4.9). Curved roofs may
adopt a semi-circular, semi-elliptic or ogival
(Gothic arch) shape, among others. The dif-
ficulty to mount glass on curved structures
has limited its use until the appearance of
semi-rigid plastic panels or flexible films.
The use of wood in the structures has also
restricted the use of curved geometries, due
to the difficulty for its construction.
In the case of gable-roof greenhouses
(Fig. 4.10), the roof may be symmetrical or
asymmetrical, with great diversity of
angules, depending on the latitude and local
conditions. The flat-roof greenhouse, ini-
tially used in the parral-type greenhouse
(typical of the south of Spain), is less and
less used. The sidewalls, in single-span or
multi-span greenhouses, can be vertical or
slightly inclined, the latter being more
advantageous for their higher light trans-
missivity; its importance decreases as the
greenhouse width increases. However, the
disadvantage is that they limit the cultiva-
tion of vertically trained species by the side-
wall, as the useful height is reduced.
Other types of greenhouses that are sel-
dom used are inflatable greenhouses, in
which the cladding plastic film is kept in
(c)
(d)
(e)
(f)
Fig. 4.9.
Some common types of curved-roof
greenhouses: semi-circular (a), semi-elliptic (b),
Gothic arch or ogive (c), asymmetric (d), attached
(lean-to) single-span (e), and single tunnel (f).
position by air pressure, notably reducing
the structural elements. Fans must supply,
continuously, the necessary pressure to
keep the plastic in position. This idea
induced the use on conventional green-
house structures double clad with inflated
plastic films, by means of air pressure. The
double cover decreases the heating costs
and better withstands the wind than the sin-
gle layer, but limits the transmissivity to
radiation in relation to the simple film.
Other variants of structures have very
little commercial interest. Greenhouses made
with hanging structures to decrease the
weight of the roof have problems of resist-
ance against strong winds, and as a conse-
quence, their use has expanded very little.
