Agriculture Reference
In-Depth Information
over the greenhouse cover to decrease the
heat losses, removing them during the day-
time (Fig. 7.5), requiring a considerable
amount of labour.
Among other peculiar insulation meth-
ods, it is worth mentioning the double-wall
greenhouse prototype, whose air chamber
is filled at night with a solid insulation mat-
erial, which is removed during the day. This
is very efficient from an insulation viewpoint
but it has not been used to any great extent
due to its cost (Short and Shoh, 1981).
Similarly, new technology has
recently been developed and tested in
Canada where the cavity inside the two
layers of the double-inflated PE green-
house was filled with foam. This had sig-
nificant heat energy savings in the winter
and provided greenhouse shading/cool-
ing on demand in the summer months
(Aberkani
et al.
., 2011).
Temporary double protection is a usual
technique in Spanish greenhouses. The use
of small tunnels or floating covers, inside
the greenhouse, during the early stages of
crop development of low crops (melon,
watermelon) allows for higher temperatures
and RH to be achieved. Contact between
plants and the plastic must be avoided, to
prevent scalds in case of high temperatures
of the plastic.
Windbreaks reduce the wind pressure and
the risk of damage to the greenhouse. In
addition, they limit heat losses by air leak-
age and by convection.
Windbreaks must be taller than the green-
house and of a semi-permeable nature. Imper-
meable windbreaks generate turbulence and
the protected distance is shorter (Fig. 7.4).
The distance protected by semi-
permeable windbreaks (Fig. 7.4) ranges from
15 to 20 times their height (Van Eimern
et al
.,
1984). The orientation must be perpendicu-
lar to the dominant winds and the distance
to the greenhouse must be from two to three
times its height, in the east, west and south
sides (in the northern hemisphere) to avoid
shadows (Fig. 7.2). On the north side, it is
enough to place them at a distance of 2 or
3 m (northern hemisphere).
7.3.5
Other insulation devices
In cold areas in China, the external north-
oriented wall (in the northern hemisphere)
of greenhouses is insulated to limit the heat
losses (Fig. 7.5).
In China it is also common to use some
kind of mats, which are extended at night
(a)
Wind
H
Protected area: 6-8 times H
(b)
Wind
H
Protected area: 15-20 times H
Fig. 7.4.
Protection provided by windbreaks: (a) impermeable windbreak; (b) permeable windbreak.
H, Height.
