S o l a r – o p t i c a l P r o p e r t i e s o f V e n e t i a n B l i n d s
327
Figure 8: Cut-off angle for different profile angles of the sun and blind geometry (L
b
/D
b
ratio).
a)
b)
Figure 9 – Slats angle control for a south (a) and east-facing (b) fenestration in Lisbon on winter and
summer solstice and spring and autumn equinox for L
b
/D
b
=1.2.
At night during the winter, autumn and spring the blind is considered fully closed to save
energy, whereas during the summer it is considered fully opened to enhance night
ventilation.
Figure 8 shows that as the profile angle of sun increases, the cut-off angle decreases.
As a result, and independently of the orientation, during the winter because the profile
angle of sun is lower the cut-off angle is higher than in the rest of the year (Fig. 9). For a
south-facing fenestration during spring and autumn equinox, the cut-off angle is almost
constant along the day, as the changing of the profile angle of the sun is almost
inexistent (Fig. 9a). For an east-facing fenestration, the shape of the different curves of
Figure 9b is very similar. In fact, due to facing to the east, the blind control is executed
only between the sun rise (near 6h) and the 12h, when there is incident direct solar
radiation. Again, as the sun rises during the morning and the profile angle of sun
increases, the cut-off angle decreases. When the incident solar radiation is totally diffuse
(after 12h), the blinds are positioned to horizontal to maximize daylighting. In Figure 8 it
was shown that as the overlapping of slats gets higher (high L
b
/D
b
ratio), the cut-off
angle becomes lower, since the incoming radiation has more difficulty to pass the blind,
as described in Gomes et al. (2012).
Conclusions
This paper presented an experimental and numerical study on the solar-optical
properties of venetian blinds. A method for determining solar optical properties –
transmittance, reflectance and absorptance – of venetian blinds based on the net
-90
-60
-30
0
30
60
90
0 10 20 30 40 50 60 70 80 90
Profile angle (º)
Cut-off angle (º)
Lb/Db=1
Lb/Db=1.1
Lb/Db=1.2
-90
-60
-30
0
30
60
90
6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time of day (h)
Cut-off angle (º)
21 Mar. Lb/Db=1.2
21 Jun. Lb/Db=1.2
21 Sep. Lb/Db=1.2
21 Dec. Lb/Db=1.2
-90
-60
-30
0
30
60
90
6 7 8 9 10 11 12 13 14 15 16 17 18 19
Time of day (h)
Cut-off angle (º)
21 Mar. Lb/Db=1.2
21 Jun. Lb/Db=1.2
21 Sep. Lb/Db=1.2
21 Dec. Lb/Db=1.2
