Agriculture Reference
In-Depth Information
Windbreak
D
North
D
A
Green house
D
D: Tw o to three times the height of the windbreak
A: 2-3 m
Fig. 7.2.
The wind affects heat losses and leakage. Distances from windbreaks to the greenhouse
(northern hemisphere, medium latitude) to avoid shadows (adapted from Urban, 1997a).
mobile thermal screens (especially in heated
greenhouses); and (iii) the use of permanent
double covers, which may be either inflat-
able (when flexible films are used) or cellu-
lar in the case of rigid panels.
Other measures are: (i) the use of wind-
breaks; (ii) specifically located insulation
(e.g. the north wall, in the northern hemi-
sphere); and (iii) providing double protec-
tion as a temporary measure.
Table 7.1.
Air infiltration in closed greenhouses
(in greenhouse volumes per hour). The lowest values
correspond to conditions with no wind. (Source:
ASAe, 1984; Nelson, 1985; López
et al
., 2001.)
Greenhouse type
Air volumes h
−1
Pe: double layer
0.5-1.0
Pe: multi-tunnel
1.0-1.5
Pe: low-cost parral-type
1.3-6.0
Glass, new
0.75-1.5
Glass, old, good maintenance
1.0-2.0
Glass, old, bad maintenance
2.0-4.0
7.3.1
Inflated double cover
value of
Q
reported here is sufficient or even
high (Hanan, 1998). For more details, see
Chapter 5 (energy balances discussed in
sections 5.3 and 5.4).
The inflated double cover may reduce the
energy losses in heated greenhouses by
approximately 30%. Maintaining an insu-
lating air chamber of several centimetres
thickness, between two plastic films,
improves the insulation properties of the
cover, reducing the global heat transfer coef-
ficient (Table 5.2). The disadvantage is an
associated reduction in light. This reduc-
tion will depend on the transmissivity
7.3
Insulation Devices
The main measures to decrease heat losses
are shown in Fig. 7.3. They are: (i) the dou-
bling the external sidewalls; (ii) the use of
