Agriculture Reference
In-Depth Information
4.12
Summary
sun are responsible for ageing plastic
materials.
Rigid plastic materials are used in the
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Nowadays, the previously considered
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main function of a greenhouse (increase
of the temperature in relation to the
open field, as a consequence of the
'greenhouse effect') in some cases
remains secondary, while the 'shading
effect' or the 'windbreak effect' of the
greenhouse under specific climate con-
ditions are considered of equal or even
greater importance.
The suitability of a location for green-
form of simple or alveolar panels
and normally need a more expensive
structure than that used with flexible
films. The most commonly used rigid
materials are polyester, PC, PVC and
PMMA.
The rigidity characteristics of the cov-
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ering materials determine, to a great
extent, the shape of the greenhouse and
the covering materials. Conventionally,
glass panels have not been used in
curved roofs, but recently curved glass
panels have become available on the
market.
The main aspects to consider in the
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house cultivation is determined by its
climatic conditions (mainly tempera-
ture and radiation) as well as other fac-
tors of a socio-economic nature.
In the last few decades flexible plastic
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films, with their low weight in rela-
tion to the materials previously used
in greenhouses (glass), have resulted
in a considerable reduction in sup-
porting structures and their cost, and
have allowed for a massive expansion
in the use of greenhouses all over the
world.
The most common plastic materials
choice of a greenhouse must be:
(i) transmission to PAR, which deter-
mines the production potential; (ii) the
solidity and longevity; (iii) the func-
tionality and ease of maintenance;
and (iv) the economics of the energy
required and price.
The most common greenhouse types
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are the artisan greenhouse and the
industrial greenhouse. Artisan green-
houses use plastic films and are cheap.
In Spain, the most common type is the
artisan low-cost parral greenhouse.
Among the industrial greenhouses, the
most common are the Venlo type, which
uses glass as cladding, and multi-span
greenhouses, which usually have a
curved-roof shape or are multi-tunnel
greenhouses with flexible plastic-film
covering although they also allow for
the use of semi-rigid panels.
Among the criteria that should be con-
used in the form of flexible films as
greenhouse covering materials are
LDPEs (in their normal, long-life and
thermal variants), EVA copolymer and
plasticized PVC, although this last
material is not used very extensively
except in Asia.
Incorporation of different additives
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improves the characteristics of plastic
films. The multilayer films (formed by
coextrusion of several layers of differ-
ent materials) enable several desirable
characteristics to be combined in a sin-
gle film, which is not possible with a
single material.
A good plastic film must have high
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sidered for the design and construc-
tion of plastic-film greenhouses
(which are heavily influenced by the
climate and latitude of the location)
the maximization of the light is the
most important, as well as providing
proper insulation and sufficient venti-
lation. In addition, such greenhouses
should be structurally sound (against
wind, snow, etc.) and the film should
be easy to assemble so that it remains
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transmission to solar radiation, a
limited transmission to long-wave IR
radiation (Earth's radiation), as well
as durability in line with its thick-
ness, formulation and cost. The lon-
gevity of a plastic film will depend
on the type of solar radiation
received, as UV rays coming from the
