Agriculture Reference
In-Depth Information
Table 4.6. Characteristics of several rigid materials used as greenhouse covers (adapted from CPA,
1992).
Horticultural
glass
Reinforced
polyester
Bioriented
PVC
Double
PMMA
Double
PC
Thickness (mm)
4
1
0.9
16
6
Weight (kg m −2 )
10
1.5
1.45
5.0
1.4
Inflammability
No
easy
No
Medium
Medium
PAR transmissivity (%)
90
80-85
79-85
82
75-80
Long-wave IR transmissivity (%)
1
4
1-2
2
5
Durability (years)
20
10
10
20
10
light diffusive power. The transmissivity to
short-wave IR is low (which is an advantage
during the summer). Although it is a good
insulator, the shape of the sheets increases
the exchange surface. Its resistance to
impact is slightly higher than that of
PMMA.
Its main advantages are the light weight,
the ease of handling, its high diffusion
of light and its high resistance to impact.
Its main disadvantage is its lower trans-
missivity to diffuse radiation and its
combustibility.
Bioriented PVC is made as corrugated
sheets and is protected against UV light. Its
light transmission is similar to that of poly-
ester, although it varies depending on the
spectra. It is a better insulator than glass. It
withstands hail well, but withstands very
high temperatures poorly.
Polycarbonate (PC) is less transparent
than glass, but has a great resistance to
impact. It is used in alveolar sheets, mainly
of double-wall types but also as simple
sheets. Its light transmission decreases
when ageing. A surface coating protects it
against UV rays ensuring after 10 years,
minimum transmission and resistance val-
ues (Wacquant, 2000). It is used when there
is serious danger of hail, and in passages,
doors or as separating walls.
Methyl polymethacrylate (PMMA) is
presented as alveolar panels of double or
triple walls in thicknesses of 8-16 mm
(known as Plexiglass, Altuglass). It
behaves in a similar way to glass and
retains its properties well with time. Its
permeability to water vapour requires
some precautions in its use. Its high price
and the thermal dilatations limit its use in
commercial greenhouse; besides, it is eas-
ily combustable.
4.6
Greenhouse Construction
4.6.1
Introduction
The prime aim in constructing a greenhouse
must be obtaining higher profitability.
Therefore, the best design must be the result
of a compromise between technical require-
ments, some of which conflict with one
another,
while
obtaining
the
highest
profitability.
Obviously, the type of greenhouse that is
constructed will vary depending on the main
purpose of the greenhouse. A greenhouse
that must protect crops from the rain (rain-
shelter), as is the case in some tropical areas,
will be conceived in a different way from a
conventional greenhouse in an area with a
Mediterranean climate, or to one in the north-
ern latitudes or in semi-desert climates.
Besides, the local climate of each
region, the specific type of greenhouse
selected will depend on the bioclimate
requirements of the species to be grown (the
type of greenhouse required for vegetable
production will be different from that
required for growing ornamental species as
they have different light and heat require-
ments). The socio-economic conditions will
also affect the type of greenhouse to be cho-
sen. For instance, limits on the initial invest-
ment in many cases restricts options
available for the type of greenhouse.
 
 
 
 
 
 
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