Agriculture Reference
In-Depth Information
and (iv) the transpiration of the crop that
is grown inside the greenhouse, possibly
reaching very high values (Fig. 4.1). By
contrast, night temperatures are only slightly
increased in relation to the outside (2-4°C,
at most) and, in some cases, are lower (ther-
mal inversion).
The maximum thermal increases vary
with the latitude and, for each specific loca-
tion, with the time of the year, as the solar
radiation changes (Fig. 4.2).
To increase the low temperatures the
most usual solution is to heat the green-
house, which is not always profitable. In
some cases, a highly isolating system can
avoid the temperature decrease during the
night, as is done in the 'lean-to greenhouse'
type in China, where a special curtain of
canes and wood materials is manually
placed over the greenhouse cover at sunset,
and removed at sunrise, to avoid relevant
temperature decreases at night, a highly
labour-intensive activity.
To limit thermal excesses, the renewal
of the interior air by means of ventilation is
the classic and most economic tool (see
Chapter 8).
The hourly air renewal rate needed to
limit the temperature gradient to an accept-
able value, depending on the maximum
predictable solar radiation (Table 4.1), can
be very high (Fig. 4.3), and this may not be
attainable in practice without mechanical
ventilation or evaporative cooling.
4.4.4
Climate suitability
The fundamental requirements of the ther-
mophilic horticultural species previously
cited as candidates for out-of-season culti-
vation (e.g. tomato, pepper, melon, water-
melon) would be:
1.
A minimum global radiation of 8.5 MJ m
−2
day
−1
(equivalent to 2.34 kWh m
−2
day
−1
).
2.
Average ambient temperatures between
17 and 27°C in coastal areas, and 17-22°C
in inland areas far from the sea. This dis-
tinction is based on the higher daily thermal
oscillations observed in continental cli-
mates (around 20°C) than in marine-type
climatic regions (10°C), and by setting the
upper threshold of air temperature at 32°C
(Nisen
et al
., 1988).
Considering the usual unacceptably
high cost of active intervention (e.g. heating
systems) on the microclimate in the case of
unsophisticated greenhouses, the minimum
greenhouse temperatures are usually simi-
lar to those of the open air. The maximum
temperatures, with passive normal ventila-
tion would be in some cases around 10°C
higher than outside. This would involve an
∆
T
(°C)
20
U
= 0 m s
-1
U
= 10 m
s
-1
10
0
R
s
kcal m
-2
h
-1
W m
-2
150
450
750
175
525
875
Fig. 4.1.
example of the temperature increase (
D
T
) in a closed greenhouse, well irrigated, as a function
of the solar radiation intensity (
R
s
) and wind velocity (
U
) (adapted from Nisen
et al
., 1988).
