Environmental Engineering Reference
In-Depth Information
l
γ
l
'
−
×
sin(
α
)
+
−
tan(
h
)
×
[(
L
−
×
cos(
α
)]
c
N
c
2
cos(
h
'
)
2
s
=
N
Afternoon:
(33)
2
'
cos(
α
)
×
tan(
h
)
−
sin(
α
)
c
N
c
H
×
sin(
γ
)
N
s
=
(34)
3
tan(
h
)
N
with reference to figure 25 we can write:
Total area of absorber plate…………
A
=
L
T
×
l
(35)
Shadow area #1……………………..
A
1
=
s
3
×
l
(36)
Shadow area #2……………………..
A
2
=
s
1
× (
L
T
-
s
3
)
(37)
Shadow area #3……………………..
A
3
= (
s
2
-
s
1
) × (
L
T
-
s
3
)
(38)
The area exposed to direct solar radiation is therefore the difference between the total
area of the absorber plate,
A
, and the three shadow area, i.e.
A
4
=
A
- (
A
1
+
A
2
+
A
3
)
(39)
It is convenient to introduce the ratios
R
1
,
R
2
,
R
3
and
R
4
such that
s
×
l
3
R
=
1
A
s
×
(
L
−
s
)
R
=
1
T
3
2
A
(40)
(
s
−
s
)
×
(
L
−
s
)
2
1
T
3
R
=
3
A
(
l
−
s
)
×
(
L
−
s
)
R
=
2
T
3
4
A
It is to be noted that areas #1 and #2 are completely shadowed by solid obstacles which
does not transmit any radiation whereas area #3 (shadow due to adjacent glass tube) has an
attenuated solar radiation due to the transmittance of three laters of glass through which each
solar ray has to travel (see figure 23). It is assumed that the daily average transmittance of the
three glass layers is equal to 0.7. This assumption is suggested by the manufacturer (Sanyo)
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