Agriculture Reference
In-Depth Information
Energy-saving screens, if used only
during the night, do not need to be trans-
missive to PAR radiation. But if they are
also used during the day it is necessary to
have a good PAR transmission, besides
their
humidity and chemical agents. Additives
are usually incorporated into the plastic
materials used as screens in order to pre-
vent dust particles sticking to the screens
and to increase their shelf life.
The screen materials must have dimen-
sional stability, and not to be prone to
stretching beyond 2% (Bakker and Van
Holsteijn, 1995). They must allow for an
easy deployment and retraction.
Nowadays, screens are made using PE,
polyester or acrylic materials as the raw
materials because other materials such as
polypropylene, polyamide and cellulose
have limited use mainly due to their lower
durability (Bakker
et al
., 1995).
Regarding texture, the screens adopt the
shape of plastic films, of woven or braided
fabric, and canvas or aluminized sheets,
associating aluminium sheets with other
materials in one or both surfaces (Urban,
1997a). There are non-woven screens too,
such as agro-textiles, which are bonded
plastic filaments spun together as a fabric.
The PE sheet screens usually only last
for two cropping seasons, with the proper
thicknesses and additives (UV). The screens
built with polyester or acrylic materials may
last for more than 5 years.
Darkening screens are usually built of
black PE film and black woven tissues,
which absorb condensation. These materi-
als may be coated with aluminium, on their
inner face, to fulfil a complementary func-
tion of energy saving (Bakker
et al
., 1995).
Shading screens may incorporate white
or aluminium sheets joined to the base
material, in an open structure (with drill-
ings), to allow the passage of air. If little
shading is desired, aluminium sheets are
not used. If the screens are of the closed
type (no drillings), that do not allow the
passage of the air, they may be used to save
energy. To allow for ventilation with these
screens, they are not completely deployed
so that there are small open sections.
The energy-saving screens used at night
must be made of PE film, coated with alu-
minium, a material that has no transmis-
sivity and great reflectivity (see Chapter 5),
or canvas, either woven with aluminium
sheets or not.
insulating
and
anti-condensation
properties.
Climate control screens are a combina-
tion of shading and energy-saving screens.
Screen assembly
Depending on their mobility, screens can be
fixed, semi-fixed or mobile.
The fixed energy-saving screens
decrease the light transmission permanently
and increase the humidity of the air. The
most common are perforated films of high
transmissivity to light, to palliate these
defects, and their use is limited to short
periods in the winter.
Semi-fixed screens decrease the prob-
lems of fixed screens but do not avoid them,
so the use of mobile screens has expanded,
with different systems employed to move
the screens (rolling, sliding, folded sheets)
(Fig. 8.16).
Mobile screens are usually extended
horizontally, although sometimes the shad-
ing ones are implemented parallel to the
roof cover. When folded they must keep
shading to the minimum. If the folded
screen adopts a north-south orientation it
will distribute the shadows more uniformly
than in the other direction. The insect exclu-
sion screens are usually extended vertically
or inclined, covering the vents.
Nowadays, there are several automatic
mechanisms to deploy and retract the screens
(traction wires, displacement bars, rolling
tubes) which allow for different implementa-
tion options, depending on the greenhouse
structure. In windy areas, an anchorage sys-
tem in the upper part of external screens
must be provided to avoid the wind lifting
them and possibly destroying them.
Materials for the screens
All types of screens must be hard wearing
and resistant to scratching, as well as to
ageing
by
UV
radiation,
temperature,
