Agriculture Reference
In-Depth Information
It is important to highlight the notorious
differences that exist, from the point of
view of radiation transmissivity, between
single-span and multi-span greenhouses
(even when spans have the same roof geom-
etry) because of the shadows between spans
(Fig. 3.4); consequently, transmissivity esti-
mates obtained in single-span greenhouses
cannot be extrapolated to multi-span types.
Perpendicular
Reflected
i
Solar radiation
3.3.3
Orientation and transmissivity
The greenhouse orientation, which is desig-
nated by the direction of the ridge line (longi-
tudinal axis of the span), at medium latitudes,
clearly influences transmissivity, in autumn
and in winter, under clear sky conditions
(when direct radiation predominates). At lati-
tudes higher than 30°, the north-south orien-
tation results in less radiation being transmitted
in winter than the east-west orientation, but
in higher uniformity (Giacomelli and Ting,
1999); as the elevation of the Sun increases
in spring, these differences notably decrease
(Fig. 3.5). In greenhouses with roofs with a
very low pitch (Fig. 3.5), the differences in
transmissivity between the east-west and the
north-south orientations are much smaller.
The uniformity of radiation in east-west
oriented greenhouses (symmetrical with a
roof pitch of around 30°) is less (on clear
days) than in north-south oriented green-
houses, but their transmissivity in autumn-
winter is higher, with differences of more
than 10% of the outdoors daily global radia-
tion around the winter solstice. However,
these differences in uniformity between
multi-span greenhouses oriented east-west
and north-south are attenuated by: (i) the
greater the height of the greenhouse (3.5-4.0
m at the gutters); (ii) the lower the span
width; and (iii) the radiation diffusion char-
acteristics of plastic films used nowadays.
Summarizing, at medium latitudes with a
predominance of clear days in autumn and
winter, such as in Mediterranean coastal areas,
the east-west orientation is preferable to the
north-south orientation, in greenhouses with
roofs with a pitch of greater than 30°, whereas
if the angle is low (e.g. low-tech greenhouses,
with around 10° roof angle) the north-south
Fig. 3.3.
The solar radiation which impinges on the
plastic film is partly transmitted (crossing the film),
reflected or absorbed by the film. The proportion of
radiation transmitted (in relation to the incident) is
known as transmissivity (or transmission) and
varies depending on the physical and chemical
characteristics of the film and on the wavelength
of the radiation. When the radiation is direct it also
depends on the angle of incidence (
i
).
On clear days, when direct radiation
predominates, the average global transmis-
sivity (fraction of global exterior radiation
that penetrates inside the greenhouse) must
be integrated as an average value for the
whole greenhouse. This is because of the
variability of radiation at different points
throughout the greenhouse caused by differ-
ential shadowing of the structural elements
of the greenhouse and of various pieces of
installed equipment (Bot, 1983).
On completely cloudy days, when all
the solar radiation is diffuse (i.e. when there
are no defined shadows) the distribution of
radiation is more homogeneous inside a
greenhouse (Baille, 1999). The average
instantaneous transmissivity of a certain
greenhouse varies throughout the day,
according to the position of the Sun in the
sky and the characteristics of the radiation;
normally, on a sunny day, it slightly increases
from dawn until noon, and decreases later
until dusk (Plates 6 and 7). When talking
about global greenhouse transmissivity, it is
normally understood as the daily average
transmissivity (proportion of daily accumu-
lated radiation which penetrates inside the
greenhouse with respect to the outside), to
distinguish it from the instantaneous values.
