The Plastic Greenhouse - Greenhouse Technology and Management - page 97

21
June

21
December

h = 76.5° 21 June
h = 29.5° 21 December

H

North

h

h

Horizontal

D

Fig. 4.23. Shadows created by an object (a tree) at noon during the summer and winter solstices. D,
Length of the shadow; H, height of the tree; h , elevation angle of the Sun (see Fig. 2.8). Latitude: 37°N.
(D = H cotangent h ).

SR

SR

29.5°
Horizontal

North

45° 45° 45°

27° 27° 27°

Fig. 4.24. A greenhouse prototype with an asymmetric roof, oriented east-west, with angles of 45° on
the south side of the roof and 27° on the north side, does not generate shadows between spans at noon
in the south of Spain (37°N), in the winter solstice. This is because the elevation angle of the Sun ( h ) is
approximately 29.5°. The higher angle of the south-facing roof (45°), where most of the radiation is captured
on these dates, induces a high transmissivity (see Fig. 4.19). SR, Solar rays (Castilla et al ., 2001).

27°

27° 27° 27° 27° 27°

5.0 m

N

3.5 m

6.0 m

Fig. 4.25. An efficient solution to achieve a good transmissivity in autumn-winter, at an affordable cost,
in low-cost multi-span greenhouses for the south of Spain is to build symmetrical spans, with roof slopes
1:2 (equivalent to angles of approximately 27°), oriented east-west. Although this greenhouse is less
transmissive in the winter solstice than the greenhouse with an asymmetric 45°/27° roof shown in
Fig. 4.24, it is easier to build, cheaper and offers less resistance to the wind (Soriano et al ., 2004a, b).