Agriculture Reference
In-Depth Information
θ
- Minimum zenith angle (noon)
approx. 60°
v - Vertical line
p - Line perpendicular to cover
SR - Solar rays
i
- Angle of incidence
q
p
p
North
q
i
SR
i
SR
v
v
45°
11°
i
= 49°
i
= 15°
Fig. 4.19.
Angle of incidence (
i
) of direct solar radiation in greenhouses oriented east-west, of low and
high roof slopes, in the south of Spain (latitude 37°N) in the winter solstice at noon.
The objective is to capture the maxi-
mum solar radiation in winter, in the case
of the single-span greenhouse, for lati-
tudes above 30°N (in the northern hemi-
sphere), as long as the roof slope is
sufficient. It will be desirable, as a general
rule, to orient the greenhouse east-west,
to maximize the capture of light in winter
(Fig. 4.20 and Plate 10). The north-south
orientation produces a more uniform dis-
tribution of radiation at different points in
the greenhouse, than the east-west orien-
tation (Fig. 4.21), especially if the roof
slope is low, and in a more marked way if
direct radiation prevails over the diffuse
(see Chapter 3).
When analysing multi-span green-
houses the problem is more complex, due to
the shadows that each span projects over
the adjacent one (Fig. 4.22).
It is desirable, for each case to study the
specific problem before installing the green-
house and to consider this together with
other aspects, as previously indicated
(shape of the roof, slopes, priority growing
season), because the design is usually more
important than the orientation.
Small deviations from the optimal ori-
entation (of the order of 15°) have very little
influence on the transmissivity, but if they
reach 30° they start to be significant.
An important aspect to consider in sin-
gle-span greenhouses, once the orientation
has been chosen, is the separation between
them to avoid shadows (Fig. 4.23). In lati-
tudes of the Mediterranean Basin a mini-
mum separation of up to 8 m is advisable (if
built at the same elevation).
Nowadays, solar radiation transmissiv-
ity models have been developed for different
types of design of greenhouse structure in
the Mediterranean, and these are an efficient
tool for the designer (Soriano
et al
., 2004b).
Summarizing, to maximize radiation,
losses due to structural elements and equip-
ment must be minimized. The site selec-
tion, orientation, shape and slope of the
roof and cladding material used are primary
aspects to capture the maximum possible
radiation inside the greenhouse, bearing in
mind that the best technical solution
(Fig. 4.24) is not always the most economi-
cally suitable (Fig. 4.25).
To maintain high transmission proper
management of the greenhouse is required.
Some of the measures that may help to
achieve the goal of maximizing the solar
radiation available for the crop in the green-
house are: (i) cleaning the cover; (ii) limit-
ing condensation on the inner surface of the
cover (as it reduces transmissivity) by means
of good climate control (which limits high
