Agriculture Reference
In-Depth Information
Film
Film
Plastic
Film
Steel
Aluminium
Plastic
Rubber
Fig. 4.11.
Different systems of how to fix the plastic film in greenhouse structures.
the drillings may rip the film. The metallic
surfaces of the structure exposed to the sun
will degrade the film as they warm up;
therefore, it is advisable to protect the film
in those contact areas with painting that
reflects the sun rays.
The ageing of films depends, besides
their intrinsic characteristics, on the condi-
tions of use, particularly the climatic
conditions such as solar radiation (inten-
sity, length and degree of exposure to the
weather). With ageing, the mechanical
characteristics deteriorate and the optical
characteristics are altered (transmission
reduction) loosing quality from the point of
view of their agronomical use.
the use of screens has allowed for an
improvement of the climate control in
sophisticated greenhouses.
Nowadays, the different types of screens
must be integrated in the greenhouses struc-
tures, minimizing the light losses.
Reasons for the use of screens
Basically, screens are used to obscure, to shade,
to decrease energy losses and for climate con-
trol. A new application is the use of insect-
proof screens which prevent insects entering
the greenhouse (Chapter 8).
Darkening screens are mobile and are
used to limit the length of the day. They
must have a minimum light transmission
(<0.1%) to achieve the short-day treatments
in crops sensitive to photoperiod (see
Chapter 9).
Shading screens are used to decrease
the light intensity and to limit direct radia-
tion, with the aim of restricting thermal
excesses, improving the quality of the pro-
duce or avoiding water stress. They can be
used inside or outside the greenhouse.
Whitewash, usual in hot climates, is a type
of low-cost fixed shading.
4.6.5
Greenhouse screens
Introduction
Screens have been traditionally used to
shade or obscure greenhouses. Later, fol-
lowing the energy crisis of the last third of
the 20th century, their use was expanded to
reduce energy costs (thermal screens), espe-
cially in heated greenhouses. More recently,
